LoongArch: Set AS_HAS_THIN_ADD_SUB as y if AS_IS_LLVM

When building kernel with "make CC=clang defconfig", LLVM Assembler is
used due to LLVM_IAS=0 is not specified, then AS_HAS_THIN_ADD_SUB is not
set, thus objtool can not be built after enable it for Clang.

config AS_HAS_THIN_ADD_SUB is to check whether -mthin-add-sub option is
available to know R_LARCH_{32,64}_PCREL are supported for GNU Assembler,
there is no such an option for LLVM Assembler. The minimal version of
Clang is 18 for building LoongArch kernel, and Clang >= 17 has already
supported R_LARCH_{32,64}_PCREL, that is to say, there is no need to
depend on AS_HAS_THIN_ADD_SUB for Clang, so just set AS_HAS_THIN_ADD_SUB
as y if AS_IS_LLVM.

Fixes: 120dd411 ("LoongArch: Only allow OBJTOOL & ORC unwinder if toolchain supports -mthin-add-sub")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>