Get the default panic hook for ICEs, from std::panic::set_hook.

[eddyb]

Tested by adding -Ztreat-err-as-bug to test_rustc_flags in tests/src/main.rs and observing that the output does include the panic information printed by the default panic hook (i.e. std's).

Background: since the rustup in #716, we've had broken ICEs - they would show this message, and the lines after that, but none of the panic information that is supposed to precede it (and RUST_BACKTRACE=1 did nothing):

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/EmbarkStudios/rust-gpu/issues/new

note: rustc 1.56.0-nightly (ae90dcf02 2021-08-09) running on x86_64-unknown-linux-gnu

That's because of an interaction between rust-lang/rust#85640 and our own use of report_ice - thankfully, @bjorn3 provided a solution that doesn't require upstream changes, in the form of an aspect of std::panic::take_hook() I missed: you can keep calling it and it will return std's default panic hook, as opposed to crashing and/or returning some kind of noop closure.

This is documented on std::panic::take_hook as such:

If no custom hook is registered, the default hook will be returned.

Knowing that, it's easy to see why you can just call it more than once to both clear any existing panic hooks and get std's default panic hook - but I went a bit farther to try and guard against strange mishaps.