LoongArch: Split the function loongarch_cpu_cpp_builtins into two functions.

LoongArch: Split the function loongarch_cpu_cpp_builtins into two functions.

Split the implementation of the function loongarch_cpu_cpp_builtins into two parts:
  1. Macro definitions that do not change (only considering 64-bit architecture)
  2. Macro definitions that change with different compilation options.

gcc/ChangeLog:

	* config/loongarch/loongarch-c.cc (builtin_undef): New macro.
	(loongarch_cpu_cpp_builtins): Split to loongarch_update_cpp_builtins
	and loongarch_define_unconditional_macros.
	(loongarch_def_or_undef): New functions.
	(loongarch_define_unconditional_macros): Likewise.
	(loongarch_update_cpp_builtins): Likewise.