Reland "[CMake][LoongArch] Add LoongArch to LLVM_ALL_TARGETS so it is built by default"

This follows the [[ https://discourse.llvm.org/t/rfc-promoting-the-loongarch-backend-from-experimental-to-official/67506/ | RFC ]].

Follow-on commits will add appropriate release notes changes etc.

Submit this now and in a minimal form so there is reasonable time before
16.0.0 is branched to resolve any issues arising from e.g. the backend
being exposed on different compiler/sanitizer setups.

The current builder for LoongArch is on the [[ https://lab.llvm.org/staging/#/builders/236 | staging area ]].

Reviewed By: jyknight, MaskRay, echristo, myhsu, tstellar, arsenm

Differential Revision: https://reviews.llvm.org/D141191