LoongArch: add model attribute

LoongArch: add model attribute

A linker script and/or a section attribute may locate some object
specially, so we need to handle the code model for such objects
differently than the -mcmodel setting. This happens when the Linux
kernel loads a module with per-CPU variables.

Add an attribute to override the code model for a specific variable.

gcc/ChangeLog:

	* config/loongarch/loongarch-protos.h (loongarch_symbol_type):
	Add SYMBOL_PCREL64 and change the description for SYMBOL_PCREL.
	* config/loongarch/loongarch.cc (loongarch_attribute_table):
	New attribute table.
	(TARGET_ATTRIBUTE_TABLE): Define the target hook.
	(loongarch_handle_model_attribute): New static function.
	(loongarch_classify_symbol): Take TARGET_CMODEL_EXTREME and the
	model attribute of SYMBOL_REF_DECL into account returning
	SYMBOL_PCREL or SYMBOL_PCREL64.
	(loongarch_use_anchors_for_symbol_p): New static function.
	(TARGET_USE_ANCHORS_FOR_SYMBOL_P): Define the target hook.
	(loongarch_symbol_extreme_p): New static function.
	(loongarch_symbolic_constant_p): Handle SYMBOL_PCREL64.
	(loongarch_symbol_insns): Likewise.
	(loongarch_split_symbol_type): Likewise.
	(loongarch_split_symbol): Check SYMBOL_PCREL64 instead of
	TARGET_CMODEL_EXTREME for PC-relative addressing.
	(loongarch_print_operand_reloc): Likewise.
	* doc/extend.texi (Variable Attributes): Document new
	LoongArch specific attribute.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/attr-model-test.c: New test.
	* gcc.target/loongarch/attr-model-1.c: New test.
	* gcc.target/loongarch/attr-model-2.c: New test.
	* gcc.target/loongarch/attr-model-diag.c: New test.