libc: Integrate strrchr in libc ...

Change-Id: I3ddc07cb02d73cd7614af7a5b21827aae155f9a0
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

libc: Import strrchr from FreeBSD ...

Imported from lib/libc/string/strrchr.c from commit:

59fd2fb

Change-Id: I898206c6f0372d4d211c149ec0fb9522d0a5b01c
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>