ARM: Add a C version of __sync_fetch_and_add_4()

It just calls atomic_add().

No need for the asm version, it doesn't need to depend on defines.

src/system/libroot/os/arch/arm/atomic.S

@@ -81,11 +81,7 @@ FUNCTION(atomic_get):
         bx	lr
 FUNCTION_END(atomic_get)
 
-FUNCTION(__sync_fetch_and_add_4):
+#endif /* ATOMIC_FUNCS_ARE_SYSCALLS */
-	bx	lr
-FUNCTION_END(__sync_fetch_and_add_4)
-
-#endif
 
 #ifndef ATOMIC64_FUNCS_ARE_SYSCALLS
 

src/system/libroot/os/atomic.c

@@ -88,3 +88,10 @@ atomic_get64(vint64 *value)
 }
 
 #endif	/* ATOMIC64_FUNCS_ARE_SYSCALLS */
+
+/* GCC compatibility: libstdc++ needs this one */
+
+extern int32_t __sync_fetch_and_add_4(int32_t *value, int32_t addValue)
+{
+	return atomic_add((vint32 *)value, addValue);
+}