runtime: Ubuntu 18.04 x64 build failing

On the current tip, I am seeing build failures on Linux x64.

The ./build.sh script output ends with a message

Build failed (exit code '139'). See log: /home/stmaclea/git/corefx/eng/common/../../artifacts/log/Debug/Build.binlog

The Build.binlog is a binary file and does not seem to contain useful information.

Early in the console output there are a series of warnings

~/git/corefx$ ./build.sh 
Downloading 'https://dot.net/v1/dotnet-install.sh'
dotnet-install: Downloading link: https://dotnetcli.azureedge.net/dotnet/Sdk/2.1.401/dotnet-sdk-2.1.401-linux-x64.tar.gz
dotnet-install: Extracting zip from https://dotnetcli.azureedge.net/dotnet/Sdk/2.1.401/dotnet-sdk-2.1.401-linux-x64.tar.gz
dotnet-install: Adding to current process PATH: `/home/stmaclea/git/corefx/.dotnet`. Note: This change will be visible only when sourcing script.
dotnet-install: Installation finished successfully.
/home/stmaclea/git/corefx/.dotnet/sdk/2.1.401/MSBuild.dll /nologo -maxcpucount /m -verbosity:m /v:minimal /bl:/home/stmaclea/git/corefx/eng/common/../../artifacts/log/Debug/ToolsetRestore.binlog /clp:Summary /clp:None /nr:true /p:__ToolsetLocationOutputFile=/home/stmaclea/git/corefx/eng/common/../../artifacts/toolset/1.0.0-beta.18565.8.txt /t:__WriteToolsetLocation /home/stmaclea/git/corefx/eng/common/../../artifacts/toolset/restore.proj

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:00.23

/home/stmaclea/git/corefx/.dotnet/sdk/2.1.401/MSBuild.dll /nologo -maxcpucount /m -verbosity:m /v:minimal /bl:/home/stmaclea/git/corefx/eng/common/../../artifacts/log/Debug/Build.binlog /clp:Summary /nr:true /p:Configuration=Debug /p:Projects=/home/stmaclea/git/corefx/eng/common/../../*.sln /p:RepoRoot=/home/stmaclea/git/corefx/eng/common/../.. /p:Restore=true /p:Build=true /p:Rebuild=false /p:Test=false /p:Pack=false /p:IntegrationTest=false /p:PerformanceTest=false /p:Sign=false /p:Publish=false /p:ContinuousIntegrationBuild=false /home/stmaclea/.nuget/packages/microsoft.dotnet.arcade.sdk/1.0.0-beta.18565.8/tools/Build.proj
  Restoring packages for /home/stmaclea/.nuget/packages/microsoft.dotnet.arcade.sdk/1.0.0-beta.18565.8/tools/Tools.proj...
  Generating MSBuild file /home/stmaclea/git/corefx/artifacts/toolset/Common/Tools.proj.nuget.g.props.
  Generating MSBuild file /home/stmaclea/git/corefx/artifacts/toolset/Common/Tools.proj.nuget.g.targets.
  Restore completed in 231.43 ms for /home/stmaclea/.nuget/packages/microsoft.dotnet.arcade.sdk/1.0.0-beta.18565.8/tools/Tools.proj.
  Restoring packages for /home/stmaclea/git/corefx/external/netstandard/netstandard.depproj...
  Generating MSBuild file /home/stmaclea/git/corefx/artifacts/obj/netstandard/netstandard/netstandard.depproj.nuget.g.props.
  Generating MSBuild file /home/stmaclea/git/corefx/artifacts/obj/netstandard/netstandard/netstandard.depproj.nuget.g.targets.
  Restore completed in 31.45 ms for /home/stmaclea/git/corefx/external/netstandard/netstandard.depproj.
/home/stmaclea/.nuget/packages/microsoft.build.tasks.git/1.0.0-beta-63401-01/build/Microsoft.Build.Tasks.Git.targets(20,5): warning : The type initializer for 'LibGit2Sharp.Core.NativeMethods' threw an exception. [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]
/home/stmaclea/.nuget/packages/microsoft.build.tasks.git/1.0.0-beta-63401-01/build/Microsoft.Build.Tasks.Git.targets(20,5): warning :    at LibGit2Sharp.Core.NativeMethods.git_buf_free(GitBuf buf) [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]
/home/stmaclea/.nuget/packages/microsoft.build.tasks.git/1.0.0-beta-63401-01/build/Microsoft.Build.Tasks.Git.targets(20,5): warning :    at LibGit2Sharp.Core.Proxy.ConvertPath(Func`2 pathRetriever) [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]
/home/stmaclea/.nuget/packages/microsoft.build.tasks.git/1.0.0-beta-63401-01/build/Microsoft.Build.Tasks.Git.targets(20,5): warning :    at LibGit2Sharp.Repository.Discover(String startingPath) [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]
/home/stmaclea/.nuget/packages/microsoft.build.tasks.git/1.0.0-beta-63401-01/build/Microsoft.Build.Tasks.Git.targets(20,5): warning :    at Microsoft.Build.Tasks.Git.GitOperations.LocateRepository(String directory) in /_/src/Microsoft.Build.Tasks.Git.Operations/GitOperations.cs:line 26 [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]
/home/stmaclea/.nuget/packages/microsoft.build.tasks.git/1.0.0-beta-63401-01/build/Microsoft.Build.Tasks.Git.targets(20,5): warning :    at Microsoft.Build.Tasks.Git.RepositoryTasks.LocateRepository(LocateRepository task) in /_/src/Microsoft.Build.Tasks.Git.Operations/RepositoryTasks.cs:line 58 [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]
/home/stmaclea/.nuget/packages/microsoft.sourcelink.common/1.0.0-beta-63401-01/build/Microsoft.SourceLink.Common.targets(53,5): warning : Source control information is not available - the generated source link is empty. [/home/stmaclea/git/corefx/external/netstandard/netstandard.depproj]