LoongArch: Allow -mcmodel=extreme and model attribute with -mexplicit-relocs=auto

LoongArch: Allow -mcmodel=extreme and model attribute with -mexplicit-relocs=auto

There seems no real reason to require -mexplicit-relocs=always for
-mcmodel=extreme or model attribute.  As the linker does not know how to
relax a 3-operand la.local or la.global pseudo instruction, just emit
explicit relocs for SYMBOL_PCREL64, and under TARGET_CMODEL_EXTREME also
SYMBOL_GOT_DISP.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_explicit_relocs_p):
	Return true for SYMBOL_PCREL64.  Return true for SYMBOL_GOT_DISP
	if TARGET_CMODEL_EXTREME.
	(loongarch_split_symbol): Check for la_opt_explicit_relocs !=
	EXPLICIT_RELOCS_NONE instead of TARGET_EXPLICIT_RELOCS.
	(loongarch_print_operand_reloc): Likewise.
	(loongarch_option_override_internal): Likewise.
	(loongarch_handle_model_attribute): Likewise.
	* doc/invoke.texi (-mcmodel=extreme): Update the compatibility
	between it and -mexplicit-relocs=.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/attr-model-3.c: New test.
	* gcc.target/loongarch/attr-model-4.c: New test.
	* gcc.target/loongarch/func-call-extreme-3.c: New test.
	* gcc.target/loongarch/func-call-extreme-4.c: New test.