gdb: tweak scoped_disable_commit_resumed uses when resuming all threads in non-stop

gdb: tweak scoped_disable_commit_resumed uses when resuming all threads in non-stop

When doing "continue -a" in non-stop mode, each thread is individually
resumed while the commit resumed state is enabled.  This forces the
target to commit each resumption immediately, instead of being able to
batch things.

The reason is that there is no scoped_disable_commit_resumed around the
loop over threads in continue_1, when "non_stop && all_threads" is true.
Since the proceed function is called once for each thread, the
scoped_disable_commit_resumed in proceed therefore forces commit-resumed
between each thread resumption.  Add the necessary
scoped_disable_commit_resumed in continue_1 to avoid that.

I looked at the MI side of things, the function exec_continue, and found
that it was correct.  There is a similar iteration over threads, and
there is a scoped_disable_commit_resumed at the function scope.  This is
not wrong, but a bit more than we need.  The branches that just call
continue_1 do not need it, as continue_1 takes care of disabling commit
resumed.  So, move the scoped_disable_commit_resumed to the inner scope
where we iterate on threads and proceed them individually.

Here's an example debugging a multi-threaded program attached by
gdbserver (debug output trimmed for brevity):

    $ ./gdb -nx -q --data-directory=data-directory -ex "set non-stop" -ex "tar rem :1234"
    (gdb) set debug remote
    (gdb) set debug infrun
    (gdb) c -a
    Continuing.
    [infrun] proceed: enter
      [infrun] scoped_disable_commit_resumed: reason=proceeding
      [remote] Sending packet: $vCont;c:p14388.14388#90
      [infrun] reset: reason=proceeding
      [infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed for target remote
      [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed for target remote
    [infrun] proceed: exit
    [infrun] proceed: enter
      [infrun] scoped_disable_commit_resumed: reason=proceeding
      [remote] Sending packet: $vCont;c:p14388.1438a#b9
      [infrun] reset: reason=proceeding
      [infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed for target remote
      [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed for target remote
    [infrun] proceed: exit
    ... and so on for each thread ...

Notice how we send one vCont;c for each thread.  With the patch applied, we
send a single vCont;c at the end:

    [infrun] scoped_disable_commit_resumed: reason=continue all threads in non-stop
    [infrun] proceed: enter
      [infrun] scoped_disable_commit_resumed: reason=proceeding
      [infrun] reset: reason=proceeding
    [infrun] proceed: exit
    [infrun] clear_proceed_status_thread: Thread 85790.85792
    [infrun] proceed: enter
      [infrun] scoped_disable_commit_resumed: reason=proceeding
      [infrun] reset: reason=proceeding
    [infrun] proceed: exit
    ... proceeding threads individually ...
    [infrun] reset: reason=continue all threads in non-stop
    [infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed for target remote
    [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed for target remote
    [remote] Sending packet: $vCont;c#a8

Change-Id: I331dd2473c5aa5114f89854196fed2a8fdd122bb