LoongArch: Rename ISA_BASE_LA64V100 to ISA_BASE_LA64

LoongArch: Rename ISA_BASE_LA64V100 to ISA_BASE_LA64

LoongArch ISA manual v1.10 suggests that software should not depend on
the ISA version number for marking processor features.  The ISA version
number is now defined as a collective name of individual ISA evolutions.
Since there is a independent ISA evolution mask now, we can drop the
version information from the base ISA.

gcc/ChangeLog:

	* config/loongarch/genopts/loongarch-strings: Rename.
	* config/loongarch/genopts/loongarch.opt.in: Same.
	* config/loongarch/loongarch-cpu.cc: Same.
	* config/loongarch/loongarch-def.cc: Same.
	* config/loongarch/loongarch-def.h: Same.
	* config/loongarch/loongarch-opts.cc: Same.
	* config/loongarch/loongarch-opts.h: Same.
	* config/loongarch/loongarch-str.h: Same.
	* config/loongarch/loongarch.opt: Same.