MIPS: ZBOOT: implement stack protector in compressed boot phase

This patch implements the stack protector code in MIPS compressed boot
phase based on the same code added to arm in commit
8779657d "stackprotector: Introduce
CONFIG_CC_STACKPROTECTOR_STRONG" by Kees Cook <keescook@chromium.org>

Signed-off-by: Ben Chan <benchan@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Olof Johansson <olofj@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/7175/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>