abort2: Generate a core dump

Call sigexit rather than exit1 so that a core is generated.

If running the SIGABRT handler is desired, this would need to use
kern_psignal() instead. In that case a userspace wrapper in libc
would be needed to force an exit if the handler doesn't exit. Given
that abort2(2)'s intended use case is when userland is in a
sufficiently bad state such that it can't safely call syslog(3) before
abort(3), a userspace abort2(3) wrapper in libc might be dubious.

Reviewed by: Olivier Certner <olce.freebsd@certner.fr>, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42163