fix build regression on ARM for ISA levels less than v5

commit 06fbefd100 (first included in
release 1.1.17) introduced this regression.

patch by Adrian Bunk. it fixes the regression in all cases, but
spuriously prevents use of the clz instruction on very old compiler
versions that don't define __ARM_ARCH. this may be fixed in a more
general way at some point in the future. it also omits thumb1 logic
since building as thumb1 code is currently not supported.

arch/arm/atomic_arch.h

@@ -82,9 +82,13 @@ static inline void a_crash()
 		: : : "memory");
 }
 
+#if __ARM_ARCH >= 5
+
 #define a_clz_32 a_clz_32
 static inline int a_clz_32(uint32_t x)
 {
 	__asm__ ("clz %0, %1" : "=r"(x) : "r"(x));
 	return x;
 }
+
+#endif