cmd/5c, cmd/5g, cmd/5l: fix cross compilation failure on darwin

Fixes #3708.

The fix to allow 5{c,g,l} to compile under clang 3.1 broke cross
compilation on darwin using the Apple default compiler on 10.7.3.

This failure was introduced in 9b455eb64690.

This has been tested by cross compiling on darwin/amd64 to linux/arm using

* gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)
* clang version 3.1 (branches/release_31)

As well as on linux/arm using

* gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)
* Ubuntu clang version 3.0-6ubuntu3 (tags/RELEASE_30/final) (based on LLVM 3.0)
* Debian clang version 3.1-4 (branches/release_31) (based on LLVM 3.1)

R=consalus, rsc
CC=golang-dev
https://golang.org/cl/6307058

src/cmd/5c/list.c

@@ -140,7 +140,7 @@ Dconv(Fmt *fp)
 {
 	char str[STRINGSZ];
 	Adr *a;
-	char *op;
+	const char *op;
 	int v;
 
 	a = va_arg(fp->args, Adr*);

src/cmd/5g/list.c

@@ -83,7 +83,7 @@ int
 Dconv(Fmt *fp)
 {
 	char str[STRINGSZ];
-	char *op;
+	const char *op;
 	Addr *a;
 	int i;
 	int32 v;

src/cmd/5l/list.c

@@ -161,7 +161,7 @@ int
 Dconv(Fmt *fp)
 {
 	char str[STRINGSZ];
-	char *op;
+	const char *op;
 	Adr *a;
 	int32 v;