Makefile: Add loongarch target flag for Clang compilation

The LoongArch kernel is 64-bit and built with the soft-float ABI,
hence the loongarch64-linux-gnusf target. (The "libc" part can affect
the codegen of libcalls: other arches do not use a bare-metal target,
and currently the only fully supported libc on LoongArch is glibc
anyway.)

See: https://lore.kernel.org/loongarch/CAKwvOdnimxv8oJ4mVY74zqtt1x7KTMrWvn2_T9x22SFDbU6rHQ@mail.gmail.com/


Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>