cmd/cgo: fix C.CString for strings containing null terminators under gccgo

Previously we used strndup(3) to implement C.CString for gccgo. This
is wrong because strndup assumes the string to be null terminated,
and stops at the first null terminator. Instead, use malloc
and memmove to create a copy of the string, as we do in the
gc implementation.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/96790047

src/cmd/cgo/out.go

@@ -1225,7 +1225,10 @@ struct __go_string __go_byte_array_to_string(const void* p, intgo len);
 struct __go_open_array __go_string_to_byte_array (struct __go_string str);
 
 const char *_cgoPREFIX_Cfunc_CString(struct __go_string s) {
-	return strndup((const char*)s.__data, s.__length);
+	char *p = malloc(s.__length+1);
+	memmove(p, s.__data, s.__length);
+	p[s.__length] = 0;
+	return p;
 }
 
 struct __go_string _cgoPREFIX_Cfunc_GoString(char *p) {