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/lib/libc/libc.mk b/lib/libc/libc.mk
index e1b5560..93d30d0 100644
--- a/lib/libc/libc.mk
+++ b/lib/libc/libc.mk
@@ -12,6 +12,7 @@
 			memcmp.c			\
 			memcpy.c			\
 			memmove.c			\
+			memrchr.c			\
 			memset.c			\
 			printf.c			\
 			putchar.c			\