[OpenMP] Fix clang-cl build error regarding TSX intrinsics

Fix for https://bugs.llvm.org/show_bug.cgi?id=49339

The CMake check for the RTM intrinsics needs the -mrtm flag to be set
during the test. This way clang-cl correctly detects it has the
_xbegin() intrinsic. Otherwise, the CMake check fails.

Differential Revision: https://reviews.llvm.org/D97413

(cherry picked from commit e83380fc)