gdb: call target_follow_exec when "set follow-exec-mode" is "same"

gdb: call target_follow_exec when "set follow-exec-mode" is "same"

target_follow_exec is currently only called in the "follow-exec-mode ==
new" branch of follow_exec, not the "follow-exec-mode == same" branch.
I think it would make sense to call it regardless of the mode to let
targets do some necessary handling.

This is needed in the context of rocm-gdb [1], where a target is pushed
on top of the linux-nat target.  On exec, it needs to do some
bookkeeping, close some file descriptors / handles that were related to
the process pre-exec and open some new ones for the process post-exec.

However, by looking at the only in-tree implementation of
target_ops::follow_exec, remote_target::follow_exec, I found that it
would be useful for the extended-remote target too, to align its
behavior with native debugging (although I think that behavior is not
very user-friendly, see PR 27745 [2]).

Using two programs, one (let's call it "execer") that execs the other
(let's call it "execee"), with native:

    $ ./gdb -q -nx --data-directory=data-directory ./execer
    Reading symbols from ./execer...
    (gdb) r
    Starting program: /home/simark/build/binutils-gdb/gdb/execer
    I am execer
    process 1495622 is executing new program: /home/simark/build/binutils-gdb/gdb/execee
    I am execee
    [Inferior 1 (process 1495622) exited normally]
    (gdb) r
    Starting program: /home/simark/build/binutils-gdb/gdb/execee
    I am execee
    [Inferior 1 (process 1495626) exited normally]

And now with gdbserver (some irrelevant output lines removed for brevity):

    $ ./gdbserver --once --multi :1234
    ...

    $ ./gdb -q -nx --data-directory=data-directory ./execer -ex "set remote exec-file /home/simark/build/binutils-gdb/gdb/execer" -ex "tar ext :1234"
    Reading symbols from ./execer...
    Remote debugging using :1234
    (gdb) r
    Starting program: /home/simark/build/binutils-gdb/gdb/execer
    process 1495724 is executing new program: /home/simark/build/binutils-gdb/gdb/execee
    [Inferior 1 (process 1495724) exited normally]
    (gdb) r
    `target:/home/simark/build/binutils-gdb/gdb/execee' has disappeared; keeping its symbols.
    Starting program: target:/home/simark/build/binutils-gdb/gdb/execee
    warning: Build ID mismatch between current exec-file target:/home/simark/build/binutils-gdb/gdb/execee
    and automatically determined exec-file target:/home/simark/build/binutils-gdb/gdb/execer
    exec-file-mismatch handling is currently "ask"
    Reading /home/simark/build/binutils-gdb/gdb/execer from remote target...
    Load new symbol table from "target:/home/simark/build/binutils-gdb/gdb/execer"? (y or n)

When handling the exec, GDB updates the exec-file of the inferior to be
the execee.  This means that a subsequent "run" will run the execee, not
the original executable (execer).

remote_target::follow_exec is meant to update the "remote exec-file",
which is the file on the remote system that will be executed if you
"run" the inferior, to the execee as well.  However, this is not called
when follow-exec-mode is same, because target_follow_exec is not called
in this branch.  As a result, GDB thinks the inferior is executing
execee but the remote side is really executing execer, hence the
mismatch message.

By calling target_follow_exec in the "same" branch of the follow_exec
function, we ensure that everybody agrees, and we get the same behavior
with the extended-remote target as we get with the native target, the
execee is executed on the second run:

    $ ./gdbserver --once --multi :1234
    ...

    $ ./gdb -q -nx --data-directory=data-directory ./execer -ex "set remote exec-file /home/simark/build/binutils-gdb/gdb/execer" -ex "tar ext :1234"
    Reading symbols from ./execer...
    Remote debugging using :1234
    (gdb) r
    Starting program: /home/simark/build/binutils-gdb/gdb/execer
    process 1501445 is executing new program: /home/simark/build/binutils-gdb/gdb/execee
    [Inferior 1 (process 1501445) exited normally]
    (gdb) r
    `target:/home/simark/build/binutils-gdb/gdb/execee' has disappeared; keeping its symbols.
    Starting program: target:/home/simark/build/binutils-gdb/gdb/execee
    [Inferior 1 (process 1501447) exited normally]
    (gdb)

This scenario is tested in gdb.base/foll-exec-mode.exp, and in fact this
patch fixes the test for me when using
--target_board=native-extended-gdbserver.

gdb/ChangeLog:

	* infrun.c (follow_exec): Call target_follow_fork when
	follow-exec-mode is same.
	* target.h (target_follow_fork): Improve doc.

[1] https://github.com/ROCm-Developer-Tools/ROCgdb
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=27745

Change-Id: I4ee84a875e39bf3f8eaf3e6789a4bfe23a2a430e