Fixed bug 4683 - SDL_atomic infinite recursion on armv6/armv5 w/ thumb

The real problem is that SDL_atomic.c was built in thumb mode instead of ARM mode, which is required to use the mcr instruction on ARM platforms. Added a compiler error to catch this case so we don't generate code that does infinite recursion.

I also added a potentially better way to handle things on Linux ARM platforms, based on comments in the Chromium headers, which we can try out after 2.0.10 ships.

src/atomic/SDL_atomic.c

@@ -289,6 +289,10 @@ SDL_AtomicGetPtr(void **a)
 #endif
 }
 
+#ifdef __thumb__
+#error This file should be built in arm mode so the mcr instruction is available for memory barriers
+#endif
+
 void
 SDL_MemoryBarrierReleaseFunction(void)
 {