LoongArch: Implement option save/restore

LTO option streaming and target attributes both require per-function
target configuration, which is achieved via option save/restore.

We implement TARGET_OPTION_{SAVE,RESTORE} to switch the la_target
context in addition to other automatically maintained option states
(via the "Save" option property in the .opt files).

Tested on loongarch64-linux-gnu without regression.

	PR target/113233

gcc/ChangeLog:

	* config/loongarch/genopts/loongarch.opt.in: Mark options with
	the "Save" property.
	* config/loongarch/loongarch.opt: Same.
	* config/loongarch/loongarch-opts.cc: Refresh -mcmodel= state
	according to la_target.
	* config/loongarch/loongarch.cc: Implement TARGET_OPTION_{SAVE,
	RESTORE} for the la_target structure; Rename option conditions
	to have the same "la_" prefix.
	* config/loongarch/loongarch.h: Same.