[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 e83380f)

Author: Peyton, Jonathan L

openmp/runtime/cmake/config-ix.cmake

@@ -172,6 +172,10 @@ if (IA32 OR INTEL64)
       }
       int main() { int a = __kmp_umwait(0, 1000); return a; }")
   check_cxx_source_compiles("${source_code}" LIBOMP_HAVE_WAITPKG_INTRINSICS)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  if (LIBOMP_HAVE_MRTM_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -mrtm")
+  endif()
   set(source_code "// check for attribute rtm and rtm intrinsics
       #ifdef IMMINTRIN_H
       #include <immintrin.h>
@@ -188,6 +192,7 @@ if (IA32 OR INTEL64)
       int main() { int a = __kmp_xbegin(); return a; }")
   check_cxx_source_compiles("${source_code}" LIBOMP_HAVE_RTM_INTRINSICS)
   set(CMAKE_REQUIRED_DEFINITIONS)
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endif()
 
 # Find perl executable