Support: normalize the default triple on Unix

This will fix cross-compiling buildbots (e.g. cygwin).  This is in the same vein
as SVN r205070.  Apply this to fix the cross-compiling scenario, even though the
preferred solution is to update the build system to normalize the embedded
triple rather than perform this at runtime every time.  This is meant to tide us
over until that approach is fleshed out and applied.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205120 91177308-0d34-0410-b5e6-96231b3b80d8