add non-asm version for gcc

mult/imult64.c

@@ -1,4 +1,4 @@
-#ifdef __x86_64__
+#if defined(__x86_64__) && defined(__OPTIMIZE__)
 
 /* WARNING: this only works if compiled with -fomit-frame-pointer */
 void imult64() {

mult/umult64.c

@@ -21,6 +21,16 @@ void umult64() {
 
 #include "safemult.h"
 
+#ifdef __GNUC__
+
+int umult64(uint64 a,uint64 b,uint64* c) {
+  __uint128_t x=((__uint128_t)a)*b;
+  if ((*c=(uint64)x) != x) return 0;
+  return 1;
+}
+
+#else
+
 /* return 1 for overflow, 0 for ok */
 int umult64(uint64 a,uint64 b,uint64* c) {
   uint32 ahi=a>>32;
@@ -46,3 +56,5 @@ int umult64(uint64 a,uint64 b,uint64* c) {
 }
 
 #endif
+
+#endif