LoongArch: Enable switchable target

LoongArch: Enable switchable target

This patch fixes the back-end context switching in cases where functions
should be built with their own target contexts instead of the
global one, such as LTO linking and functions with target attributes (TBD).

	PR target/113233

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_reg_init):
	Reinitialize the loongarch_regno_mode_ok cache.
	(loongarch_option_override): Same.
	(loongarch_save_restore_target_globals): Restore target globals.
	(loongarch_set_current_function): Restore the target contexts
	for functions.
	(TARGET_SET_CURRENT_FUNCTION): Define.
	* config/loongarch/loongarch.h (SWITCHABLE_TARGET): Enable
	switchable target context.
	* config/loongarch/loongarch-builtins.cc (loongarch_init_builtins):
	Initialize all builtin functions at startup.
	(loongarch_expand_builtin): Turn assertion of builtin availability
	into a test.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Define condition loongarch_sx_as.
	* gcc.dg/lto/pr113233_0.c: New test.