runtime: Net Core 3.0 bootstrap failure for c7d48ca

Commit c7d48ca7732b7717e84d8375588d83866104ef58 builds a really broken corefx when using 3.0preview5 as the initial bootstrap source.

joshua@nova:~/netcore/corefxϟ uname -a
Linux nova 4.19.0-4-amd64 dotnet/corefx#1 SMP Debian 4.19.28-2 (2019-03-15) x86_64 GNU/Linux
joshua@nova:~/netcore/corefxϟPATH=/home/joshua/netcore/3.0preview5:$PATH ./build.sh -restore -build -buildtests -test

generates hunderds of test failures of the form

<failure exception-type="System.TypeInitializationException">
      <message><![CDATA[System.TypeInitializationException : The type initializer for 'System.PlatformDetection' threw an exception.\n---- System.FormatException : Input string was not in a correct format.]]></message>
      <stack-trace><![CDATA[   at System.PlatformDetection.get_IsNonZeroLowerBoundArraySupported() in /home/joshua/netcore/corefx/src/CoreFx.Private.TestUtilities/src/System/PlatformDetection.cs:line 119
   at System.Text.RegularExpressions.Tests.GroupCollectionTests.ICollection_CopyTo_Invalid() in /home/joshua/netcore/corefx/src/System.Text.RegularExpressions/tests/GroupCollectionTests.cs:line 119
----- Inner Stack Trace -----
   at System.Number.ThrowOverflowOrFormatException(ParsingStatus status, TypeCode type) in /_/src/System.Private.CoreLib/shared/System/Number.Parsing.cs:line 1929
   at System.Number.ParseInt32(ReadOnlySpan`1 value, NumberStyles styles, NumberFormatInfo info) in /_/src/System.Private.CoreLib/shared/System/Number.Parsing.cs:line 218
   at System.Int32.Parse(String s) in /_/src/System.Private.CoreLib/shared/System/Int32.cs:line 107
   at System.PlatformDetection.ToVersion(String versionString) in /home/joshua/netcore/corefx/src/CoreFx.Private.TestUtilities/src/System/PlatformDetection.Unix.cs:line 165
   at System.PlatformDetection..cctor() in /home/joshua/netcore/corefx/src/CoreFx.Private.TestUtilities/src/System/PlatformDetection.Unix.cs:line 84]]></stack-trace>
    </failure>