LoongArch: Fix multiarch tuple canonization

LoongArch: Fix multiarch tuple canonization

Multiarch tuple will be coded in file or directory names in
multiarch-aware distros, so one ABI should have only one multiarch
tuple.  For example, "--target=loongarch64-linux-gnu --with-abi=lp64s"
and "--target=loongarch64-linux-gnusf" should both set multiarch tuple
to "loongarch64-linux-gnusf".  Before this commit,
"--target=loongarch64-linux-gnu --with-abi=lp64s --disable-multilib"
will produce wrong result (loongarch64-linux-gnu).

A recent LoongArch psABI revision mandates "loongarch64-linux-gnu" to be
used for -mabi=lp64d (instead of "loongarch64-linux-gnuf64") for some
non-technical reason [1].  Note that we cannot make
"loongarch64-linux-gnuf64" an alias for "loongarch64-linux-gnu" because
to implement such an alias, we must create thousands of symlinks in the
distro and doing so would be completely unpractical.  This commit also
aligns GCC with the revision.

Tested by building cross compilers with --enable-multiarch and multiple
combinations of --target=loongarch64-linux-gnu*, --with-abi=lp64{s,f,d},
and --{enable,disable}-multilib; and run "xgcc --print-multiarch" then
manually verify the result with eyesight.

[1]: https://github.com/loongson/LoongArch-Documentation/pull/80

gcc/ChangeLog:

	* config.gcc (triplet_abi): Set its value based on $with_abi,
	instead of $target.
	(la_canonical_triplet): Set it after $triplet_abi is set
	correctly.
	* config/loongarch/t-linux (MULTILIB_OSDIRNAMES): Make the
	multiarch tuple for lp64d "loongarch64-linux-gnu" (without
	"f64" suffix).