LoongArch: Reimplement multilib build option handling.

LoongArch: Reimplement multilib build option handling.

Library build options from --with-multilib-list used to be processed with
*self_spec, which missed the driver's initial canonicalization.  This
caused limitations on CFLAGS override and the use of driver-only options
like -m[no]-lsx.

The problem is solved by promoting the injection rules of --with-multilib-list
options to the first element of DRIVER_SELF_SPECS, to make them execute before
the canonialization.  The library-build options are also hard-coded in
the driver and can be used conveniently by the builders of other non-gcc
libraries via the use of -fmultiflags.

Bootstrapped and tested on loongarch64-linux-gnu.

ChangeLog:

	* config-ml.in: Remove unneeded loongarch clause.
	* configure.ac: Register custom makefile fragments mt-loongarch-*
	for loongarch targets.
	* configure: Regenerate.

config/ChangeLog:

	* mt-loongarch-mlib: New file.  Pass -fmultiflags when building
	target libraries (FLAGS_FOR_TARGET).
	* mt-loongarch-elf: New file.
	* mt-loongarch-gnu: New file.

gcc/ChangeLog:

	* config.gcc: Pass the default ABI via TM_MULTILIB_CONFIG.
	* config/loongarch/loongarch-driver.h: Invoke MLIB_SELF_SPECS
	before the driver canonicalization routines.
	* config/loongarch/loongarch.h: Move definitions of CC1_SPEC etc.
	to loongarch-driver.h
	* config/loongarch/t-linux: Move multilib-related definitions to
	t-multilib.
	* config/loongarch/t-multilib: New file.  Inject library build
	options obtained from --with-multilib-list.
	* config/loongarch/t-loongarch: Same.