Fix the CMake build when setting MACOSX_DEPLOYMENT_TARGET

If we don't reset CMAKE_OSX_DEPLOYMENT_TARGET, it will end up in
CFLAGS together with the -mmacosx-version-min that we set,
and the compiler errors about the conflict.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@215468 91177308-0d34-0410-b5e6-96231b3b80d8