libc: add memrchr

This function scans a string backwards from the end for the first
instance of a character.

Change-Id: I46b21573ed25a0ff222eac340e1e1fb93b040763
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>

lib/libc/libc.mk

@@ -12,6 +12,7 @@ LIBC_SRCS	:=	$(addprefix lib/libc/,		\
 			memcmp.c			\
 			memcpy.c			\
 			memmove.c			\
+			memrchr.c			\
 			memset.c			\
 			printf.c			\
 			putchar.c			\

lib/libc/memrchr.c

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2019, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <string.h>
+
+#undef memrchr
+
+void *memrchr(const void *src, int c, size_t len)
+{
+	const unsigned char *s = src + (len - 1);
+
+	while (len--) {
+		if (*s == (unsigned char)c) {
+			return (void*) s;
+		}
+
+		s--;
+	}
+
+	return NULL;
+}