Fix ARM AAL on Morello

The spelling of inline assembler constraints on Morello would be
different, but Jessica Clarke points out that we should just be using
the builtin when available, so do that instead.

Co-authored-by: Jessica Clarke <jrtc27@jrtc27.com>

src/aal/aal_arm.h

@@ -48,12 +48,12 @@ namespace snmalloc
     {
 #ifdef _MSC_VER
       __prefetch(ptr);
-#else
-#  ifdef SNMALLOC_VA_BITS_64
+#elif __has_builtin(__builtin_prefetch) && !defined(SNMALLOC_NO_AAL_BUILTINS)
+      __builtin_prefetch(ptr);
+#elif defined(SNMALLOC_VA_BITS_64)
       __asm__ volatile("prfm pldl1keep, [%0]" : "=r"(ptr));
-#  else
+#else
       __asm__ volatile("pld\t[%0]" : "=r"(ptr));
-#  endif
 #endif
     }
   };