Define a_clz_32 for musl so that it doesn't use a software version. (#171)

Author: sunfishcode

libc-top-half/musl/arch/wasm32/atomic_arch.h

@@ -5,3 +5,7 @@
 #define a_barrier() (__sync_synchronize())
 #define a_cas(p, t, s) (__sync_val_compare_and_swap((p), (t), (s)))
 #define a_crash() (__builtin_trap())
+#define a_clz_32 __builtin_clz
+#define a_clz_64 __builtin_clzll
+#define a_ctz_32 __builtin_ctz
+#define a_ctz_64 __builtin_ctzll