[openmp] Only try to add the -mrtm flag on i386/x86_64

Clang does accept the flag for any architecture, but prints a
warning:

    clang-16: warning: argument unused during compilation: '-mrtm' [-Wunused-command-line-argument]

This flag is documented as an x86 specific flag, so don't try to
add it for other architectures, to silence this warning.

Differential Revision: https://reviews.llvm.org/D137744

openmp/runtime/cmake/LibompHandleFlags.cmake

@@ -42,7 +42,9 @@ function(libomp_get_cxxflags cxxflags)
   libomp_append(flags_local /GS LIBOMP_HAVE_GS_FLAG)
   libomp_append(flags_local /EHsc LIBOMP_HAVE_EHSC_FLAG)
   libomp_append(flags_local /Oy- LIBOMP_HAVE_OY__FLAG)
-  libomp_append(flags_local -mrtm LIBOMP_HAVE_MRTM_FLAG)
+  if(${IA32} OR ${INTEL64})
+    libomp_append(flags_local -mrtm LIBOMP_HAVE_MRTM_FLAG)
+  endif()
   # Intel(R) C Compiler flags
   libomp_append(flags_local /Qsafeseh LIBOMP_HAVE_QSAFESEH_FLAG)
   libomp_append(flags_local -Qoption,cpp,--extended_float_types LIBOMP_HAVE_EXTENDED_FLOAT_TYPES_FLAG)