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

src/string/memrchr.c

@@ -0,0 +1,12 @@
+#include <string.h>
+#include "libc.h"
+
+void *__memrchr(const void *m, int c, size_t n)
+{
+	const unsigned char *s = m;
+	c = (unsigned char)c;
+	while (n--) if (s[n]==c) return (void *)(s+n);
+	return 0;
+}
+
+weak_alias(__memrchr, memrchr);