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>
diff --git a/include/lib/libc/string.h b/include/lib/libc/string.h
index ee6eeac..de20f61 100644
--- a/include/lib/libc/string.h
+++ b/include/lib/libc/string.h
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 /*
- * Portions copyright (c) 2018, ARM Limited and Contributors.
+ * Portions copyright (c) 2018-2019, ARM Limited and Contributors.
  * All rights reserved.
  */
 
@@ -23,6 +23,7 @@
 int strcmp(const char *s1, const char *s2);
 int strncmp(const char *s1, const char *s2, size_t n);
 void *memchr(const void *src, int c, size_t len);
+void *memrchr(const void *src, int c, size_t len);
 char *strchr(const char *s, int c);
 void *memset(void *dst, int val, size_t count);
 size_t strlen(const char *s);