gdb: add inferior_forked observable

gdb: add inferior_forked observable

In the upcoming patch to support fork in the amd-dbgapi target, the
amd-dbgapi target will need to be notified of fork events through an
observer, to attach itself (attach in the amd-dbgapi sense, not ptrace
sense) to the new inferior / process.

The reason that this can't be done through target_ops::follow_fork is
that the amd-dbgapi target isn't pushed on the inferior's target stack
right away.  It attaches itself to the process and only pushes itself on
its target stack if and when the inferior initializes the ROCm runtime.

If an inferior that is not using the ROCm runtime forks, we want to be
notified of it, so we can attach to the child, and catch if the child
starts using the ROCm runtime.

So, add a new observable and notify it in follow_fork_inferior.  It will
be used later in this series.

Change-Id: I67fced5a9cba6d5da72b9c7ea1c8397644ca1d54
Reviewed-By: Pedro Alves <pedro@palves.net>