Update PR tests slightly and when what runs where (#5805)

* Don't run LLDB tests on PRs

These take an extra minute or so, so only run them on the full test
suite of a merge instead of on all PRs as well.

* Add a test for the x64 isa files

This guarantees that if cranelift's x64 backend is modified that the
tests will be run on a PR, even if other backends were also modified.

.github/workflows/main.yml

@@ -448,7 +448,7 @@ jobs:
     - run: |
         sudo apt-get update && sudo apt-get install -y gdb lldb llvm
         cargo test test_debug_dwarf -- --ignored --test-threads 1
-      if: matrix.os == 'ubuntu-latest' && matrix.target == ''
+      if: matrix.os == 'ubuntu-latest' && matrix.target == ''&& needs.determine.outputs.run-full
       env:
         RUST_BACKTRACE: 1