Recognize loongarch32, loongarch64, and loongarchx32

* config.guess (loongarch32:Linux:*:*, loongarch64:Linux:*:*,
loongarchx32:Linux:*:*): Recognize.
* config.sub (loongarch32, loongarch64, loongarchx32): Likewise.
* doc/config.guess.1: Regenerate.
* doc/config.sub.1: Likewise.
* testsuite/config-guess.data: Add test cases for loongarch32,
loongarch64, and loongarchx32.
* testsuite/config-sub.data (loongarch32, loongarch64, loongarchx32):
Add test cases.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>