Use __builtin_ctz and __builtin_clz in dlmalloc (WebAssembly#401)

Co-authored-by: Ben Smith <[email protected]>

Author: TerrorJack

dlmalloc/src/malloc.c

@@ -2839,7 +2839,7 @@ static size_t traverse_and_check(mstate m);
 #define treebin_at(M,i)     (&((M)->treebins[i]))
 
 /* assign tree index for size S to variable I. Use x86 asm if possible  */
-#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__) || defined(__wasm__))
 #define compute_tree_index(S, I)\
 {\
   unsigned int X = S >> TREEBIN_SHIFT;\
@@ -2942,7 +2942,7 @@ static size_t traverse_and_check(mstate m);
 
 /* index corresponding to given bit. Use x86 asm if possible */
 
-#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__) || defined(__wasm__))
 #define compute_bit2idx(X, I)\
 {\
   unsigned int J;\