LoongArch: Implement target pragma.

LoongArch: Implement target pragma.

The target pragmas defined correspond to the target function attributes.

This implementation is derived from AArch64.

gcc/ChangeLog:

	* config/loongarch/loongarch-protos.h
	(loongarch_reset_previous_fndecl):  Add function declaration.
	(loongarch_save_restore_target_globals): Likewise.
	(loongarch_register_pragmas): Likewise.
	* config/loongarch/loongarch-target-attr.cc
	(loongarch_option_valid_attribute_p): Optimize the processing
	of attributes.
	(loongarch_pragma_target_parse): New functions.
	(loongarch_register_pragmas): Likewise.
	* config/loongarch/loongarch.cc
	(loongarch_reset_previous_fndecl): New functions.
	(loongarch_set_current_function): When the old_tree is the same
	as the new_tree, the rules for using registers, etc.,
	are set according to the option values to ensure that the
	pragma can be processed correctly.
	* config/loongarch/loongarch.h (REGISTER_TARGET_PRAGMAS):
	Define macro.
	* doc/extend.texi: Supplemental Documentation.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/arch-func-attr-1.c: Add '#pragma'.
	* gcc.target/loongarch/cmodel-func-attr-1.c: Likewise.
	* gcc.target/loongarch/lasx-func-attr-1.c: Likewise.
	* gcc.target/loongarch/lsx-func-attr-1.c: Likewise.
	* gcc.target/loongarch/strict_align-func-attr-1.c: Likewise.
	* gcc.target/loongarch/strict_align-func-attr-2.c: Likewise.
	* gcc.target/loongarch/vector-func-attr-1.c: Likewise.
	* gcc.target/loongarch/arch-pragma-attr-1.c: Likewise.
	* gcc.target/loongarch/cmodel-pragma-attr-1.c: New test.
	* gcc.target/loongarch/lasx-pragma-attr-1.c: New test.
	* gcc.target/loongarch/lasx-pragma-attr-2.c: New test.
	* gcc.target/loongarch/lsx-pragma-attr-1.c: New test.
	* gcc.target/loongarch/lsx-pragma-attr-2.c: New test.
	* gcc.target/loongarch/strict_align-pragma-attr-1.c: New test.
	* gcc.target/loongarch/strict_align-pragma-attr-2.c: New test.
	* gcc.target/loongarch/vector-pragma-attr-1.c: New test.
	* gcc.target/loongarch/pragma-push-pop.c: New test.