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>

scripts/Makefile.clang

@@ -4,6 +4,7 @@
 CLANG_TARGET_FLAGS_arm		:= arm-linux-gnueabi
 CLANG_TARGET_FLAGS_arm64	:= aarch64-linux-gnu
 CLANG_TARGET_FLAGS_hexagon	:= hexagon-linux-musl
+CLANG_TARGET_FLAGS_loongarch	:= loongarch64-linux-gnusf
 CLANG_TARGET_FLAGS_m68k		:= m68k-linux-gnu
 CLANG_TARGET_FLAGS_mips		:= mipsel-linux-gnu
 CLANG_TARGET_FLAGS_powerpc	:= powerpc64le-linux-gnu