LoongArch: Specify -m32/-m64 explicitly for 32BIT/64BIT

LoongArch: Specify -m32/-m64 explicitly for 32BIT/64BIT

Clang/LLVM build needs -m32/-m64 to switch triple variants (i.e. the
--target=xxx parameter). Otherwise we get build errors for CONFIG_32BIT.

GCC doesn't support -m32/-m64 now, but maybe support in future, so use
cc-option to specify them.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604232041.ESJDwVG4-lkp@intel.com/
Suggested-by: Nathan Chancellor <nathan@kernel.org
Tested-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>