x64/LJ_GC64: Fix warning for DUALNUM build.

src/vm_x64.dasc

@@ -1469,7 +1469,7 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov [BASE-16], CFUNC:RD
   |  mov [BASE-8], TMPR
   |.if DUALNUM
-  |  mov64 RD, ((int64_t)LJ_TISNUM<<47)
+  |  mov64 RD, ((uint64_t)LJ_TISNUM<<47)
   |  mov [BASE], RD
   |.else
   |  mov qword [BASE], 0