Improve remote attach round-trips without btrace

Improve remote attach round-trips without btrace

For remotes which do not support btrace at all, we can save several
round trips for each thread.  This is especially significant when your
remote is a kernel with 100s or 1000s of threads and latency is
intercontinental.

Previously, with target, remote, and infrun debugging enabled, one
might see:

    Sending packet: $Hg18aee#43...Ack
    Packet received: OK
    Sending packet: $Hg186f7#eb...Ack
    Packet received: OK
    remote:target_xfer_partial (24, , 0x805454000, 0x0, 0x0, 4096) = -1, 0

repeated for all non-exited threads.

Afterwards, if the remote does not specify 'qXfer:btrace-conf:read+'
in qSupported stub features, these unnecessary thread switches are
avoided.

gdb/ChangeLog:

	* remote.c (remote_target::remote_btrace_maybe_reopen): Avoid
	unnecessary thread walk if remote doesn't support the packet.