llvm-test.py: Use msys make when running regression tests on Windows
llvm-test.py: Use msys make when running regression tests on Windows

Add the msys directory to the path so that 'make check' works.

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/161513002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12758 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client