LoongArch: Use explicit relocs for TLS access with -mexplicit-relocs=auto

The linker does not know how to relax TLS access for LoongArch, so let's
emit machine instructions with explicit relocs for TLS.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_explicit_relocs_p):
	Return true for TLS symbol types if -mexplicit-relocs=auto.
	(loongarch_call_tls_get_addr): Replace TARGET_EXPLICIT_RELOCS
	with la_opt_explicit_relocs != EXPLICIT_RELOCS_NONE.
	(loongarch_legitimize_tls_address): Likewise.
	* config/loongarch/loongarch.md (@tls_low<mode>): Remove
	TARGET_EXPLICIT_RELOCS from insn condition.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/explicit-relocs-auto-tls-ld-gd.c: New
	test.
	* gcc.target/loongarch/explicit-relocs-auto-tls-le-ie.c: New
	test.