[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