runtime: Perf: DateTime.Now 3x slower on Linux

Perf test:

[Benchmark]
public void GetNow()
{
    DateTime dt;
    foreach (var iteration in Benchmark.Iterations)
        using (iteration.StartMeasurement())
            for (int i = 0; i < 10000; i++)
            {
                dt = DateTime.Now; dt = DateTime.Now; dt = DateTime.Now;
                dt = DateTime.Now; dt = DateTime.Now; dt = DateTime.Now;
                dt = DateTime.Now; dt = DateTime.Now; dt = DateTime.Now;
            }
}

Linux results:

        <performance runid="System.Runtime.Tests.Perf_DateTime.GetNow-Linux" etl="/home/ianha/uptonogood/results/System.Runtime.Tests.Perf_DateTime.GetNow-Linux.csv">
          <metrics>
            <Duration displayName="Duration" unit="msec" />
          </metrics>
          <iterations>
            <iteration index="0" Duration="65.38629999756813" />
            <iteration index="1" Duration="61.40879999101162" />
            <iteration index="2" Duration="64.916299998760223" />
            <iteration index="3" Duration="55.876999989151955" />
            <iteration index="4" Duration="55.115500003099442" />
            <iteration index="5" Duration="59.215800002217293" />
            <iteration index="6" Duration="55.531300008296967" />
            <iteration index="7" Duration="55.105000004172325" />
            <iteration index="8" Duration="56.883499994874" />
            <iteration index="9" Duration="56.871900007128716" />
            <iteration index="10" Duration="55.777700006961823" />
            <iteration index="11" Duration="54.972599998116493" />
            <iteration index="12" Duration="54.644500002264977" />
            <iteration index="13" Duration="91.604200005531311" />
            <iteration index="14" Duration="56.039199993014336" />
            <iteration index="15" Duration="59.405699998140335" />
            <iteration index="16" Duration="59.423000007867813" />
            <iteration index="17" Duration="60.069600000977516" />
          </iterations>
        </performance>

Windows results:

        <performance runid="System.Runtime.Tests.Perf_DateTime.GetNow-WindowsCore" etl="D:\git\corefx\bin\tests\Windows_NT.AnyCPU.Release\system.runtime.tests\dnxcore50\System.Runtime.Tests.Perf_DateTime.GetNow-WindowsCore.etl">
          <metrics>
            <Duration displayName="Duration" unit="msec" />
          </metrics>
          <iterations>
            <iteration index="0" Duration="20.669701876065005" />
            <iteration index="1" Duration="22.393721341775745" />
            <iteration index="2" Duration="20.51631631751502" />
            <iteration index="3" Duration="20.602132401387848" />
            <iteration index="4" Duration="19.901348566971251" />
            <iteration index="5" Duration="19.689801941610312" />
            <iteration index="6" Duration="19.730571709031551" />
            <iteration index="7" Duration="19.689801941610312" />
            <iteration index="8" Duration="19.4235154886627" />
            <iteration index="9" Duration="20.746394655340168" />
            <iteration index="10" Duration="19.552097062837674" />
            <iteration index="11" Duration="19.373622416643684" />
            <iteration index="12" Duration="19.429217554036313" />
            <iteration index="13" Duration="19.524442045775572" />
            <iteration index="14" Duration="19.493935996026721" />
            <iteration index="15" Duration="19.33256754595368" />
            <iteration index="16" Duration="19.180322400478303" />
            <iteration index="17" Duration="19.447179059963219" />
            <iteration index="18" Duration="19.438911065171396" />
            <iteration index="19" Duration="19.340550437476736" />
            <iteration index="20" Duration="19.799281596783544" />
            <iteration index="21" Duration="19.912752697718361" />
            <iteration index="22" Duration="19.690657251416269" />
            <iteration index="23" Duration="20.093223066793144" />
            <iteration index="24" Duration="20.79058566198546" />
            <iteration index="25" Duration="20.538269269203283" />
            <iteration index="26" Duration="19.73883970382326" />
            <iteration index="27" Duration="20.204128238309977" />
            <iteration index="28" Duration="19.688946631804129" />
            <iteration index="29" Duration="19.661291614742368" />
            <iteration index="30" Duration="19.702916691969449" />
            <iteration index="31" Duration="19.440906788052189" />
            <iteration index="32" Duration="19.573194704719981" />
            <iteration index="33" Duration="19.61653040155943" />
            <iteration index="34" Duration="20.462716903002956" />
            <iteration index="35" Duration="19.710044273686663" />
            <iteration index="36" Duration="19.620806950589667" />
            <iteration index="37" Duration="19.614534678678638" />
            <iteration index="38" Duration="19.191156324688109" />
            <iteration index="39" Duration="19.816957999441911" />
            <iteration index="40" Duration="23.061433197025508" />
            <iteration index="41" Duration="21.294363137743858" />
            <iteration index="42" Duration="21.79243854812853" />
            <iteration index="43" Duration="20.098069822360685" />
            <iteration index="44" Duration="20.516031214246368" />
            <iteration index="45" Duration="21.206551330990351" />
            <iteration index="46" Duration="20.443900087270094" />
            <iteration index="47" Duration="20.521448176351214" />
            <iteration index="48" Duration="20.199281482742435" />
            <iteration index="49" Duration="20.074121147791629" />
            <iteration index="50" Duration="19.988305063918688" />
          </iterations>
        </performance>