It is possible to link libunwind and use the normal backtrace code, but it fails
to symbolize stack traces. I investigated and could get the list of instruction
pointers and symbol names, but I'm not sure how to use the dwarf info to map
from instruction pointer to source location. In any case, fixing this is
probably not a high priority.
See https://github.com/rust-lang/rust/issues/131738
previously, we only held a lock for printing the backtrace itself. since all threads were printing to the same file descriptor, that meant random output in the default panic hook would be interleaved with the backtrace. now, we hold the lock for the full duration of the hook, and the output is ordered.
This adds the `only-apple`/`ignore-apple` compiletest directive, and
uses that basically everywhere instead of `only-macos`/`ignore-macos`.
Some of the updates in `run-make` are a bit redundant, as they use
`ignore-cross-compile` and won't run on iOS - but using Apple in these
is still more correct, so I've made that change anyhow.
