[Chapel Merge] Re-deprecate --llvm and fix linux64-incr testing

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Branch: refs/heads/master
Revision: 77a006c
Author: mppf
Log Message:

Merge pull request #17885 from mppf/follow-up-17800

Re-deprecate --llvm and fix linux64-incr testing

Follow-up to #17800.

  • Sets CHPL_TARGET_COMPILER=gnu for linux64-incr (since LLVM backend
    does not yet support incremental compilation)
  • De-de-deprecates --llvm (reverts #17880). The real fix here is to stop
    using --llvm or CHPL_LLVM_CODEGEN in the test configs which I have
    done in separate PRs.

Reviewed by @Maxrimus - thanks!