SDL_atomic.h: __ARM_ARCH is not always defined for an arm platform

e.g. riscos toolchain
(cherry picked from commit 8105f1d)

include/SDL_atomic.h

@@ -240,7 +240,7 @@ typedef void (*SDL_KernelMemoryBarrierFunc)();
 /* "REP NOP" is PAUSE, coded for tools that don't know it by that name. */
 #if (defined(__GNUC__) || defined(__clang__)) && (defined(__i386__) || defined(__x86_64__))
     #define SDL_CPUPauseInstruction() __asm__ __volatile__("pause\n")  /* Some assemblers can't do REP NOP, so go with PAUSE. */
-#elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
+#elif (defined(__arm__) && defined(__ARM_ARCH) && __ARM_ARCH >= 7) || defined(__aarch64__)
     #define SDL_CPUPauseInstruction() __asm__ __volatile__("yield" ::: "memory")
 #elif (defined(__powerpc__) || defined(__powerpc64__))
     #define SDL_CPUPauseInstruction() __asm__ __volatile__("or 27,27,27");