std panicking: Provide panic::always_abort

std panicking: Provide panic::always_abort

We must change the atomic read on panic entry to `Acquire`, to pick up
a possible an `always_panic` on another thread.

We add `count` to the names of panic_count::get and ::is_zaero,
because now there is another reason why panic ought to maybe abort.
Renaming these ensures that we have checked every call site to ensure
that they don't need further adjustment.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Co-authored-by: Mara Bos <m-ou.se@m-ou.se>