fix build failure on arm building C code in thumb1 mode

a fully thumb1 build is not supported because some asm files are
incompatible with thumb1, but apparently it works to compile the C
code as thumb1

commit 06fbefd100 caused this regression
but introducing use of the clz instruction, which is not supported in
arm mode prior to v5, and not supported in thumb prior to thumb2
(v6t2). commit 1b9406b03c fixed the
issue only for arm mode pre-v5 but left thumb1 broken.

arch/arm/atomic_arch.h

@@ -83,7 +83,7 @@ static inline void a_crash()
 		: : : "memory");
 }
 
-#if __ARM_ARCH >= 5
+#if __ARM_ARCH >= 5 && (!__thumb__ || __thumb2__)
 
 #define a_clz_32 a_clz_32
 static inline int a_clz_32(uint32_t x)