LoongArch: Cleanup __builtin_constant_p() checking for cpu_has_*

In the current configuration, cpu_has_lsx and cpu_has_lasx cannot be
constants. So cleanup the __builtin_constant_p() checking to reduce the
complexity.

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