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
Try only two slashes as that seems to be working elsewhere in this
testcase.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187487 91177308-0d34-0410-b5e6-96231b3b80d8
Run an experiment to try to understand the windows failure better by
replacing one variable with the regex. This won't fix anything, but will
hopefully shed light on the nature of the failure.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187488 91177308-0d34-0410-b5e6-96231b3b80d8