libc: Import strrchr from FreeBSD
libc: Import strrchr from FreeBSD

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

59fd2fb98e4cc7e9bfc89598e28e21d405fd470c

Change-Id: I898206c6f0372d4d211c149ec0fb9522d0a5b01c
Signed-off-by: Antonio Nino Diaz <5c24ff39be768bf31f4deee44042a11772cfd02a@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware