util/u_process: Add util_get_process_exec_path for macOS.
Fixes: f8f1413070ae ("util/u_process: add util_get_process_exec_path")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2682
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
cc: @pepp