LoongArch: Implement target attribute.

LoongArch: Implement target attribute.

Add function attributes support for LoongArch.

Currently, the following items are supported:

        __attribute__ ((target ("{no-}strict-align")))
        __attribute__ ((target ("cmodel=")))
        __attribute__ ((target ("arch=")))
        __attribute__ ((target ("tune=")))
        __attribute__ ((target ("{no-}lsx")))
        __attribute__ ((target ("{no-}lasx")))

This implementation is derived from AArch64.

gcc/ChangeLog:

	* attr-urls.def: Regenerate.
	* config.gcc: Add loongarch-target-attr.o to extra_objs.
	* config/loongarch/loongarch-protos.h
	(loongarch_option_valid_attribute_p): Function declaration.
	(loongarch_option_override_internal): Likewise.
	* config/loongarch/loongarch.cc
	(loongarch_option_override_internal): Delete the modifications
	to target_option_default_node and target_option_current_node.
	(loongarch_set_current_function): Add annotation information.
	(loongarch_option_override): add assignment operations to
	target_option_default_node and target_option_current_node.
	(TARGET_OPTION_VALID_ATTRIBUTE_P): Define.
	* config/loongarch/t-loongarch: Add compilation of target file
	loongarch-target-attr.o.
	* doc/extend.texi: Add description information of LoongArch
	Function Attributes.
	* config/loongarch/loongarch-target-attr.cc: New file.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/arch-func-attr-1.c: New test.
	* gcc.target/loongarch/cmodel-func-attr-1.c: New test.
	* gcc.target/loongarch/lasx-func-attr-1.c: New test.
	* gcc.target/loongarch/lasx-func-attr-2.c: New test.
	* gcc.target/loongarch/lsx-func-attr-1.c: New test.
	* gcc.target/loongarch/lsx-func-attr-2.c: New test.
	* gcc.target/loongarch/strict_align-func-attr-1.c: New test.
	* gcc.target/loongarch/strict_align-func-attr-2.c: New test.
	* gcc.target/loongarch/vector-func-attr-1.c: New test.
	* gcc.target/loongarch/attr-check-error-message.c: New test.