LoongArch: Add ifdefs to fix LSX and LASX related warnings

There exist some warnings when building kernel if CONFIG_CPU_HAS_LBT is
set but CONFIG_CPU_HAS_LSX and CONFIG_CPU_HAS_LASX are not set. In this
case, there are no definitions of _restore_lsx & _restore_lasx and there
are also no definitions of kvm_restore_lsx & kvm_restore_lasx in fpu.S
and switch.S respectively, just add some ifdefs to fix these warnings.

  AS      arch/loongarch/kernel/fpu.o
arch/loongarch/kernel/fpu.o: warning: objtool: unexpected relocation symbol type in .rela.discard.func_stack_frame_non_standard: 0
arch/loongarch/kernel/fpu.o: warning: objtool: unexpected relocation symbol type in .rela.discard.func_stack_frame_non_standard: 0

  AS [M]  arch/loongarch/kvm/switch.o
arch/loongarch/kvm/switch.o: warning: objtool: unexpected relocation symbol type in .rela.discard.func_stack_frame_non_standard: 0
arch/loongarch/kvm/switch.o: warning: objtool: unexpected relocation symbol type in .rela.discard.func_stack_frame_non_standard: 0

  MODPOST Module.symvers
ERROR: modpost: "kvm_restore_lsx" [arch/loongarch/kvm/kvm.ko] undefined!
ERROR: modpost: "kvm_restore_lasx" [arch/loongarch/kvm/kvm.ko] undefined!

Cc: stable@vger.kernel.org # 6.9+
Fixes: cb8a2ef0 ("LoongArch: Add ORC stack unwinder support")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202408120955.qls5oNQY-lkp@intel.com/


Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>