LoongArch: Use explicit relocs for GOT access when -mexplicit-relocs=auto and...

LoongArch: Use explicit relocs for GOT access when -mexplicit-relocs=auto and LTO during a final link with linker plugin

If we are performing LTO for a final link and linker plugin is enabled,
then we are sure any GOT access may resolve to a symbol out of the link
unit (otherwise the linker plugin will tell us the symbol should be
resolved locally and we'll use PC-relative access instead).

Produce machine instructions with explicit relocs instead of la.global
for better scheduling.

gcc/ChangeLog:

	* config/loongarch/loongarch-protos.h
	(loongarch_explicit_relocs_p): Declare new function.
	* config/loongarch/loongarch.cc (loongarch_explicit_relocs_p):
	Implement.
	(loongarch_symbol_insns): Call loongarch_explicit_relocs_p for
	SYMBOL_GOT_DISP, instead of using TARGET_EXPLICIT_RELOCS.
	(loongarch_split_symbol): Call loongarch_explicit_relocs_p for
	deciding if return early, instead of using
	TARGET_EXPLICIT_RELOCS.
	(loongarch_output_move): CAll loongarch_explicit_relocs_p
	instead of using TARGET_EXPLICIT_RELOCS.
	* config/loongarch/loongarch.md (*low<mode>): Remove
	TARGET_EXPLICIT_RELOCS from insn condition.
	(@ld_from_got<mode>): Likewise.
	* config/loongarch/predicates.md (move_operand): Call
	loongarch_explicit_relocs_p instead of using
	TARGET_EXPLICIT_RELOCS.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/explicit-relocs-auto-lto.c: New test.