x86-asm: Accept 'q' modifier

In inline extended asm '%q1' refers to the 64bit register of operand 1.

tests/tcctest.c

@@ -2584,6 +2584,17 @@ unsigned long mconstraint_test(struct struct1231 *r)
 	             : "=&r" (ret), "=m" (a)
 	             : "m" (*(struct struct123 *)r->addr));
     return ret + a[0];
+}
+
+#ifdef __x86_64__
+int fls64(unsigned long long x)
+{
+  int bitpos = -1;
+  asm("bsrq %1,%q0"
+      : "+r" (bitpos)
+      : "rm" (x));
+  return bitpos + 1;
+}
 #endif
 
 unsigned int set;