kernel: Add core dump facility

* Add function core_dump_write_core_file(). It writes a core file for
the current thread's team. The file format is similar to that of
other OSs (i.e. ELF with PT_LOAD segments and a PT_NOTE segment), but
most of the notes are Haiku specific (infos for team, areas, images,
threads). More data will probably need to be added.
* Add team flag TEAM_FLAG_DUMP_CORE, thread flag
THREAD_FLAGS_TRAP_FOR_CORE_DUMP, and Team property coreDumpCondition,
a condition variable available while a core dump is progress. A
thread that finds its flag THREAD_FLAGS_TRAP_FOR_CORE_DUMP set before
exiting the kernel to userland calls core_dump_trap_thread(), which
blocks on the condition variable until the core dump has finished. We
need the team's threads to stop so we can get their CPU state (and
have a generally unchanging team state while writing the core file).
* Add user debugger message B_DEBUG_WRITE_CORE_FILE. It causes
core_dump_write_core_file() to be called for the team.
* Dumping core as an immediate effect of a terminal signal has not been
implemented yet, but that should be fairly straight forward.