feat(lib/libc): add strchr required by new mbedtls configs

New MBEDTLS configs added as part of SPDM requester libraries requires
strchr().

This file strchr.c is taken from Trusted Firmware-A release v2.10.

Signed-off-by: Arunachalam Ganapathy <arunachalam.ganapathy@arm.com>
Change-Id: If202c63a63be1d3b1795c0c9d8d795e2cc536b22
3 files changed