cmd/compile, cmd/compile/internal/x86: do not let regopt use CX in position independent code

We might be able to do better than this, but it's a start.

Change-Id: I80ebce9094e084a4746039106ccf1ad9c4b8bb7c
Reviewed-on: https://go-review.googlesource.com/16384Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/x86/reg.go

@@ -62,7 +62,11 @@ func regnames(n *int) []string {
 }
 
 func excludedregs() uint64 {
-	return RtoB(x86.REG_SP)
+	if gc.Ctxt.Flag_shared != 0 {
+		return RtoB(x86.REG_SP) | RtoB(x86.REG_CX)
+	} else {
+		return RtoB(x86.REG_SP)
+	}
 }
 
 func doregbits(r int) uint64 {