LoongArch: Check whether binutils supports the relax function. If supported, explicit...

LoongArch: Check whether binutils supports the relax function. If supported, explicit relocs are turned off by default.

gcc/ChangeLog:

	* config.in: Regenerate.
	* config/loongarch/genopts/loongarch.opt.in: Add compilation option
	mrelax. And set the initial value of explicit-relocs according to the
	detection status.
	* config/loongarch/gnu-user.h: When compiling with -mno-relax, pass the
	--no-relax option to the linker.
	* config/loongarch/loongarch-driver.h (ASM_SPEC): When compiling with
	-mno-relax, pass the -mno-relax option to the assembler.
	* config/loongarch/loongarch-opts.h (HAVE_AS_MRELAX_OPTION): Define macro.
	* config/loongarch/loongarch.opt: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add detection of support for binutils relax function.