MCJIT: ensure that cygwin is identified properly

Cygwin is now a proper environment rather than an OS.  This updates the MCJIT
tests to avoid execution on Cygwin.  This fixes native cygwin tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205266 91177308-0d34-0410-b5e6-96231b3b80d8