gdb: terminate upon receipt of SIGFPE

gdb: terminate upon receipt of SIGFPE

GDB's SIGFPE handling is broken, this is PR gdb/16505 and
PR gdb/17891.

We currently try to use an async event token to process SIGFPE.  So,
when a SIGFPE arrives the signal handler calls
mark_async_signal_handler then returns, effectively ignoring the
signal (for now).

The intention is that later the event loop will see that the async
token associated with SIGFPE has been marked and will call the async
handler, which just throws an error.

The problem is that SIGFPE is not safe to ignore.  Ignoring a
SIGFPE (unless it is generated artificially, e.g. by raise()) is
undefined behaviour, after ignoring the signal on many targets we
return to the instruction that caused the SIGFPE to be raised, which
immediately causes another SIGFPE to be raised, we get stuck in an
infinite loop.  The behaviour is certainly true on x86-64.

To view this behaviour I simply added some dummy code to GDB that
performed an integer divide by zero, compiled this on x86-64
GNU/Linux, ran GDB and saw GDB hang.

In this commit, I propose to remove all special handling of SIGFPE and
instead just let GDB make use of the default SIGFPE action, that is,
to terminate the process.

The only user visible change here should be:

  - If a user sends a SIGFPE to GDB using something like kill,
    previously GDB would just print an error and remain alive, now GDB
    will terminate.  This is inline with what happens if the user
    sends GDB a SIGSEGV from kill though, so I don't see this as an
    issue.

  - If a bug in GDB causes a real SIGFPE, previously the users GDB
    session would hang.  Now the GDB session will terminate.  Again,
    this is inline with what happens if GDB receives a SIGSEGV due to
    an internal bug.

In bug gdb/16505 there is mention that it would be nice if GDB did
more than just terminate when receiving a fatal signal.  I haven't
done that in this commit, but later commits will move in that
direction.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16505
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17891