PPC: Don't use callee-save registers when folding '%' operator.

src/buildvm_ppc.dasc

@@ -1337,7 +1337,7 @@ static void build_subroutines(BuildCtx *ctx)
   |2:
   |   cmplwi CARG5, 5; beq >1; bgt >2
   |  evmr CARG3, CARG2; efddiv CRET2, CARG2, CARG4; evmr RB, CARG4
-  |  mflr SAVE0; bl ->vm_floor; mtlr SAVE0
+  |  mflr RC; bl ->vm_floor; mtlr RC
   |  efdmul CRET2, CRET2, RB; efdsub CRET2, CARG3, CRET2
   |  evmergehi CRET1, CRET2, CRET2; blr
   |1: