LoongArch: Add ARCH_HAS_FORTIFY_SOURCE selection

FORTIFY_SOURCE could detect various overflows at compile and run time.
ARCH_HAS_FORTIFY_SOURCE means that the architecture can be built and run
with CONFIG_FORTIFY_SOURCE. So select it in LoongArch.

See more about this feature from commit 6974f0c4 ("include/linux/
string.h: add the option of fortified string.h functions").

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>