[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

llvm-svn: 364485