[cmake] Add LoongArch to config.guess

Necessary for native builds on LoongArch. Only Linux is added for now,
matching upstream config.git changes.

Unfortunately, the upstream config.guess is GPLv3+ since 2012, and
LLVM is without autoconf support for a long time, so the script's
licensing exception no longer works and a bump could not be trivially
done.

This is the same as the [upstream commit adding LoongArch](https://git.savannah.gnu.org/gitweb/?p=config.git;a=commitdiff;h=c8ddc8472f8efcadafc1ef53ca1d863415fddd5f),
but rewritten to match the 2011 style that our script is holding on
to.

Differential Revision: https://reviews.llvm.org/D130820