LoongArch: Add evolution features of base ISA revisions

	* config/loongarch/loongarch-def.h:
	(loongarch_isa_base_features): Declare.  Define it in ...
	* config/loongarch/loongarch-cpu.cc
	(loongarch_isa_base_features): ... here.
	(fill_native_cpu_config): If we know the base ISA of the CPU
	model from PRID, use it instead of la64 (v1.0).  Check if all
	expected features of this base ISA is available, emit a warning
	if not.
	* config/loongarch/loongarch-opts.cc (config_target_isa): Enable
	the features implied by the base ISA if not -march=native.