LoongArch: improved target configuration interface

LoongArch: improved target configuration interface

The configure script and the GCC driver are updated so that
it is easier to customize and control GCC builds for targeting
different LoongArch implementations.

* Make --with-abi obsolete, since it might cause different default ABI
  under the same target triplet, which is undesirable.  The default ABI
  is now purely decided by the target triplet.

* Support options for LoongArch SIMD extensions:
  new configure options --with-simd={none,lsx,lasx};
  new compiler option -msimd={none,lsx,lasx};
  new driver options -m[no]-l[a]sx.

* Enforce the priority of configuration paths (for <parm>={fpu,tune,simd}):
  -m<parm> > -march-implied > --with-<parm> > --with-arch-implied.

* Allow the user to control the compiler options used when building
  GCC libraries for each multilib variant via --with-multilib-list
  and --with-multilib-default.  This could become more useful when
  we have 32-bit support later.

  Example 1: the following configure option
    --with-multilib-list=lp64d/la464/mno-strict-align/msimd=lsx,lp64s/mfpu=32
                          |     |            |         |
                    -mabi=ABI  -march=ARCH  a list of other options
                  (mandatory)  (optional)     (optional)

     builds two sets of libraries:
     1. lp64d/base ABI (built with "-march=la464 -mno-strict-align -msimd=lsx")
     2. lp64s/base ABI (built with "-march=abi-default -mfpu=32")

  Example 2: the following 3 configure options

    --with-arch=loongarch64
    --with-multilib-list=lp64d,lp64f,lp64s/la464
    --with-multilib-default=fixed/mno-strict-align/mfpu=64
                             |            |           |
                        -march=ARCH   a list of other options
                         (optional)        (optional)

    is equivalent to (in terms of building libraries):

    --with-multilib-list=\
    lp64d/loongarch64/mno-strict-align/mfpu=64,\
    lp64f/loongarch64/mno-strict-align/mfpu=64,\
    lp64s/la464

  Note:
    1. the GCC driver and compiler proper does not support
       "-march=fixed". "fixed" that appear here acts as a placeholder for
       "use whatever ARCH in --with-arch=ARCH" (or the default value
       of --with-arch=ARCH if --with-arch is not explicitly configured).

    2. if the ARCH part is omitted, "-march=abi-default"
       is used for building all library variants, which
       practically means enabling the minimal ISA features
       that can support the given ABI.

ChangeLog:

	* config-ml.in: Do not build the multilib library variant
	that is duplicate with the toplevel one.

gcc/ChangeLog:

	* config.gcc: Make --with-abi= obsolete, decide the default ABI
	with target triplet.  Allow specifying multilib library build
	options with --with-multilib-list and --with-multilib-default.
	* config/loongarch/t-linux: Likewise.
	* config/loongarch/genopts/loongarch-strings: Likewise.
	* config/loongarch/loongarch-str.h: Likewise.
	* doc/install.texi: Likewise.
	* config/loongarch/genopts/loongarch.opt.in: Introduce
	-m[no-]l[a]sx options.  Only process -m*-float and
	-m[no-]l[a]sx in the GCC driver.
	* config/loongarch/loongarch.opt: Likewise.
	* config/loongarch/la464.md: Likewise.
	* config/loongarch/loongarch-c.cc: Likewise.
	* config/loongarch/loongarch-cpu.cc: Likewise.
	* config/loongarch/loongarch-cpu.h: Likewise.
	* config/loongarch/loongarch-def.c: Likewise.
	* config/loongarch/loongarch-def.h: Likewise.
	* config/loongarch/loongarch-driver.cc: Likewise.
	* config/loongarch/loongarch-driver.h: Likewise.
	* config/loongarch/loongarch-opts.cc: Likewise.
	* config/loongarch/loongarch-opts.h: Likewise.
	* config/loongarch/loongarch.cc: Likewise.
	* doc/invoke.texi: Likewise.