Remove asm multi-constraint

clang generates poor code otherwise:

https://stackoverflow.com/questions/16850309/clang-llvm-inline-assembly-multiple-constraints-with-useless-spills-reload

src/m_fixed.h

@@ -40,7 +40,7 @@
       }
       int32_t lo = a;
       int32_t hi = a >> 32;
-      asm("idivl %[divisor]" : "+a" (lo), "+d" (hi) : [divisor] "rm" (b));
+      asm("idivl %[divisor]" : "+a" (lo), "+d" (hi) : [divisor] "r" (b));
       return lo;
   }