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.