LoongArch: Don't default to -mno-explicit-relocs if -mno-relax

LoongArch: Don't default to -mno-explicit-relocs if -mno-relax

To improve Binutils compatibility we've had to backport relaxation
support.  But if a user just updates to GCC 13.3 and sticks with
Binutils 2.41, there is no reason to use -mno-explicit-relocs as the
default because we are turning off relaxation for Binutils 2.41 (it
lacks conditional branch relaxation support) anyway.

So like GCC 14, make the default of -m[no-]explicit-relocs depend on
-m[no-]relax instead of HAVE_AS_MRELAX_OPTION.  Also update the doc to
reflect the behavior change.

gcc/ChangeLog:

	* config/loongarch/genopts/loongarch.opt.in
	(TARGET_EXPLICIT_RELOCS): Init to M_OPTION_NOT_SEEN.
	* config/loongarch/loongarch.opt: Regenerate.
	* config/loongarch/loongarch.cc
	(loongarch_option_override_internal): Set the default of
	TARGET_EXPLICIT_RELOCS to HAVE_AS_EXPLICIT_RELOCS
	&& !loongarch_mrelax.
	* doc/invoke.texi (-m[no-]explicit-relocs): Update for
	LoongArch.