Output when QEMU/docker is actually used

[Susurrus]

We've been trying to debug some musl test issues, but we aren't certain if they're QEMU bugs or not since musl should be compileable on x86_64-linux for i686 and x86_64 architectures. cross doesn't output anything specific specifying when QEMU or Docker is being used, though Docker can be inferred from stdout. Having both of those output during a run would be quite useful.

[Alexhuszagh]

Maybe linux-runner (and also android-runner if #720 merges) should output diagnostic information if cross is run with the --verbose flag? We could probably export an envvar like CROSS_VERBOSE=1 and then if so, /linux-runner and /android-runner will echo the command they're running.

If we run an Android image with verbose output, it will currently report that we are using Qemu.

   Finished dev [unoptimized + debuginfo] target(s) in 0.94s
     Running `qemu-i386 -cpu n270 /target/i686-linux-android/debug/hellopp

But using a Linux runner will not:

    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `/linux-runner powerpc64le /target/powerpc64le-unknown-linux-gnu/debug/hellopp`

[Alexhuszagh]

We now have verbose diagnostic information with QEMU_STRACE, CROSS_DEBUG (specifies every command run in the runner), and you can override CROSS_TARGET_X86_64_UNKNOWN_LINUX_MUSK_RUNNER=qemu-user or CROSS_TARGET_X86_64_UNKNOWN_LINUX_MUSK_RUNNER=native or do so for any other target (or using Cross.toml below), so you can control if Qemu is used or not, and get verbose diagnostic info on Qemu's output or the scripts used in the Linux/Android runners. This is also documented on the wiki.

[target.x86_64-unknown-linux-musl]
runner = "native"