Previously, dtrace_panic() would call into some unfinished compat code
and return without panicking. This compat code is responsible for doing
a bunch of things that are handled by panic(9) in FreeBSD, so we don't
actually need it. In particular:

• we don't need to serialize calls to panic(9), since the first thing vpanic does is send an IPI to all other CPUs to put them in a busy loop, and wait for all other CPUs to acknowledge before proceeding
• we don't need to save registers, since said IPI causes all CPUs to save their registers to stoppcbs
• we don't use a dedicated panic stack

This change just exports FreeBSD's vpanic() and modifies dtrace_panic()
to call it directly.

Tried using the panic() action from a few different probes, and made sure that the kernel panicked cleanly and dumped core. In particular, 'fbt::: {panic();}' works properly on amd64.