implement memrchr (nonstandard) and optimize strrchr in terms of it

src/string/strrchr.c

@@ -1,9 +1,8 @@
 #include <string.h>
 
+void *__memrchr(const void *, int, size_t);
+
 char *strrchr(const char *s, int c)
 {
-	const char *p;
-	c = (char)c;
-	for (p=s+strlen(s); p>=s && *p!=c; p--);
-	return p>=s ? (char *)p : 0;
+	return __memrchr(s, c, strlen(s));
 }