provide optimized a_ctz_32 for arm

Provide an ARM specific a_ctz_32 helper function for architecture
versions for which it can be implemented efficiently via the "rbit"
instruction (ie all Thumb-2 capable versions of ARM v6 and above).

arch/arm/atomic_arch.h

@@ -91,4 +91,16 @@ static inline int a_clz_32(uint32_t x)
 	return x;
 }
 
+#if __ARM_ARCH_6T2__ || __ARM_ARCH_7A__ || __ARM_ARCH_7R__ || __ARM_ARCH >= 7
+
+#define a_ctz_32 a_ctz_32
+static inline int a_ctz_32(uint32_t x)
+{
+	uint32_t xr;
+	__asm__ ("rbit %0, %1" : "=r"(xr) : "r"(x));
+	return a_clz_32(xr);
+}
+
+#endif
+
 #endif