[cmake] Allow config.guess to be run with MSYS on Windows
Summary:
With r363420, config.guess can no longer be run with MSYS on Windows and this
patch should be able to fix this particular case.
Reviewers: compnerd
Reviewed By: compnerd
Subscribers: mgorny, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D63834
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364485 91177308-0d34-0410-b5e6-96231b3b80d8