gdb: follow-fork: push target and add thread in target_follow_fork

gdb: follow-fork: push target and add thread in target_follow_fork

In the context of ROCm-gdb [1], the ROCm target sits on top of the
linux-nat target.  when a process forks, it needs to carry over some
data from the forking inferior to the fork child inferior.  Ideally, the
ROCm target would implement the follow_fork target_ops method, but there
are some small problems.  This patch fixes these, which helps the ROCm
target, but also makes things more consistent and a bit nicer in
general, I believe.

The main problem is: when follow-fork-mode is "parent",
target_follow_fork is called with the parent as the current inferior.
When it's "child", target_follow_fork is called with the child as the
current inferior.  This means that target_follow_fork is sometimes
called on the parent's target stack and sometimes on the child's target
stack.

The parent's target stack may contain targets above the process target,
such as the ROCm target.  So if follow-fork-child is "parent", the ROCm
target would get notified of the fork and do whatever is needed.  But
the child's target stack, at that moment, only contains the exec and
process target copied over from the parent.  The child's target stack is
set up by follow_fork_inferior, before calling target_follow_fork.  In
that case, the ROCm target wouldn't get notified of the fork.

For consistency, I think it would be good to always call
target_follow_fork on the parent inferior's target stack.  I think it
makes sense as a way to indicate "this inferior has called fork, do
whatever is needed".  The desired outcome of the fork (whether an
inferior is created for the child, do we need to detach from the child)
can be indicated by passed parameter.

I therefore propose these changes:

 - make follow_fork_inferior always call target_follow_fork with the
   parent as the current inferior.  That lets all targets present on the
   parent's target stack do some fork-related handling and push
   themselves on the fork child's target stack if needed.

   For this purpose, pass the child inferior down to target_follow_fork
   and follow_fork implementations.  This is nullptr if no inferior is
   created for the child, because we want to detach from it.

 - as a result, in follow_fork_inferior, detach from the parent inferior
   (if needed) only after the target_follow_fork call.  This is needed
   because we want to call target_follow_fork before the parent's
   target stack is torn down.

 - hand over to the targets in the parent's target stack (including the
   process target) the responsibility to push themselves, if needed, to
   the child's target stack.  Also hand over the responsibility to the
   process target, at the same time, to create the child's initial
   thread (just like we do for follow_exec).

 - pass the child inferior to exec_on_vfork, so we don't need to swap
   the current inferior between parent and child.  Nothing in
   exec_on_vfork depends on the current inferior, after this change.

   Although this could perhaps be replaced with just having the exec
   target implement follow_fork and push itself in the child's target
   stack, like the process target does... We would just need to make
   sure the process target calls beneath()->follow_fork(...).  I'm not
   sure about this one.

gdb/ChangeLog:

	* target.h (struct target_ops) <follow_fork>: Add inferior*
	parameter.
	(target_follow_fork): Likewise.
	* target.c (default_follow_fork): Likewise.
	(target_follow_fork): Likewise.
	* fbsd-nat.h (class fbsd_nat_target) <follow_fork>: Likewise.
	(fbsd_nat_target::follow_fork): Likewise, and call
	inf_ptrace_target::follow_fork.
	* linux-nat.h (class linux_nat_target) <follow_fork>: Likewise.
	* linux-nat.c (linux_nat_target::follow_fork): Likewise, and
	call inf_ptrace_target::follow_fork.
	* obsd-nat.h (obsd_nat_target) <follow_fork>: Likewise.
	* obsd-nat.c (obsd_nat_target::follow_fork): Likewise, and call
	inf_ptrace_target::follow_fork.
	* remote.c (class remote_target) <follow_fork>: Likewise.
	(remote_target::follow_fork): Likewise, and call
	process_stratum_target::follow_fork.
	* process-stratum-target.h (class process_stratum_target)
	<follow_fork>: New.
	* process-stratum-target.c
	(process_stratum_target::follow_fork): New.
	* target-delegates.c: Re-generate.

[1] https://github.com/ROCm-Developer-Tools/ROCgdb

Change-Id: I460bd0af850f0485e8aed4b24c6d8262a4c69929