compiletest: Support aux-bin with cross compilation

Step-by-step

1. src/ci/docker/run.sh --dev armhf-gnu
2. ../x test --target arm-unknown-linux-gnueabihf tests/ui/compiletest-self-test/test-aux-bin.rs

(Watch out for the problem fixed in https://github.com/rust-lang/rust/pull/124359.)

Expected

The test is run through the existing remote-test-client / remote-test-server mechanism and passes.

Actual

It is ignored because aux-bin does not work for this case yet:

running 1 tests
test [ui] tests/ui/compiletest-self-test/test-aux-bin.rs ... ignored, ignored when cross-compiling (because aux-bin does not yet support it)

The problem

The problem is simply that the existing remote-test-client / remote-test-server mechanism has not been taught how to deal with aux-bin files.

Compare with

If you run the test without cross compilation it works:

$ ./x test tests/ui/compiletest-self-test/test-aux-bin.rs
running 1 tests
test [ui] tests/ui/compiletest-self-test/test-aux-bin.rs ... ok

Other info

I was asked here to file this issue.

Start with src/ci/docker/run.sh --dev armhf-gnu and reproduce the issue using ../x test --target arm-unknown-linux-gnueabihf tests/ui/compiletest-self-test/test-aux-bin.rs. Read the compiletest remote-test-client and remote-test-server handling alongside tests/ui/compiletest-self-test/test-aux-bin.rs. Done means the test runs through that mechanism and passes instead of being ignored during cross compilation.