gdb: register SIGBUS, SIGFPE, and SIGABRT handlers

gdb: register SIGBUS, SIGFPE, and SIGABRT handlers

Register handlers for SIGBUS, SIGFPE, and SIGABRT.  All of these
signals are setup as fatal signals that will cause GDB to terminate.
However, by passing these signals through the handle_fatal_signal
function, a user can arrange to see a backtrace when GDB
terminates (see maint set backtrace-on-fatal-signal).

In normal use of GDB there should be no user visible changes after
this commit.  Only if GDB terminates with one of the above signals
will GDB change slightly, potentially printing a backtrace before
aborting.

I've added new tests for SIGFPE, SIGBUS, and SIGABRT.