Re-enable the cross-linux test on windows after making it tolerant of
the path separator used when locating crtbegin.o.

I'll watch the bots to see if there are other issues lurking here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187486 91177308-0d34-0410-b5e6-96231b3b80d8