Bug 1496503 - Change the rust panic hook to delegate to Gecko's crash code. r=froydnj

The current rust panic hook keeps a string for the crash reporter, and
goes on calling the default rust panic hook, which prints out a crash
stack...  when RUST_BOOTSTRAP is set *and* when that works. Notably, on
both mac and Windows, it only really works for local builds, but fails
for debug builds from automation, although on automation itself, we also
do stackwalk from crash minidumps, which alleviates the problem.
Artifact debug builds are affected, though.

More importantly, C++ calls to e.g. MOZ_CRASH have a similar but
different behavior, in that they dump a stack trace on debug builds, by
default (with exceptions, see below for one). The format of those stack
traces is understood by the various fix*stack*py scripts under
tools/rb/, that are used by the various test harnesses both on
automation and locally.

Additionally, the current rust panic hook, as it calls the default rust
panic hook, ends up calling abort() on non-Windows platforms, which ends
up being verbosely redirected to mozalloc_abort per
https://dxr.mozilla.org/mozilla-central/rev/237e4c0633fda8e227b2ab3ab57e417c980a2811/memory/mozalloc/mozalloc_abort.cpp#79
which then calls MOZ_CRASH. Theoretically, /that/ would also print a
stack trace, but doesn't because currently the stack trace printing code
lives in libxul, and MOZ_CRASH only calls it when compiled from
libxul-code, which mozalloc_abort is not part of.

With this change, we make the rust panic handler call back into
MOZ_CRASH directly. This has multiple advantages:
- This is more consistent cross-platforms (Windows is not special
anymore).
- This is more consistent between C++ and rust (stack traces all look
the same, and can all be post-processed by fix*stack*py if need be)
- This is more consistent in behavior, where debug builds will show
those stack traces without caring about environment variables.
- It demangles C++ symbols in rust-initiated stack traces (for some
reason that didn't happen with the rust panic handler)

A few downsides:
- the loss of demangling for some rust symbols.
- the loss of addresses in the stacks, although they're not entirely
useful
- extra empty lines.

The first should be fixable later one. The latter two are arguably
something that should be consistent across C++ and rust, and should be
changed if necessary, independently of this patch.

Depends on D11719

Differential Revision: https://phabricator.services.mozilla.com/D11720

--HG--
extra : moz-landing-system : lando