Fix string.char() range check on x64.

src/buildvm_x86.dasc

@@ -2027,9 +2027,9 @@ static void build_subroutines(BuildCtx *ctx, int cmov, int sse)
   |  cmp NARGS:RD, 1+1;  jne ->fff_fallback	// *Exactly* 1 arg.
   |  cmp dword [BASE+4], LJ_TISNUM;  ja ->fff_fallback
   if (sse) {
-    |  cvtsd2si RC, qword [BASE]
-    |  cmp RC, 255;  ja ->fff_fallback
-    |  mov TMP2, RC
+    |  cvtsd2si RB, qword [BASE]
+    |  cmp RB, 255;  ja ->fff_fallback
+    |  mov TMP2, RB
   } else {
     |  fld qword [BASE]
     |  fistp TMP2