Fix potential hang during gdbserver testing

Fix potential hang during gdbserver testing

We're currently seeing testing of native-extended-gdbserver hang while
testing the x86_64 architecture on both Fedora 34 and Fedora Rawhide.
The test responsible for the hang is gdb.threads/fork-plus-threads.exp.

While there is clearly a problem/bug with this test on F34 and
Rawhide, it's also the case that testing should not hang.  This commit
prevents the hang by waiting with the "-nowait" flag in
close_gdbserver.

The -nowait flag is also used in the kill_wait_spawned_process proc in
gdb/testsuite/lib/gdb.exp, so there is precedent for doing this.

There are also 15 other uses of "wait -i" scattered throughout the
test suite.  While it's tempting to change these to also use the
-nowait flag, I think it might be safer to defer doing so until we
actually see a problem.

I've tested this patch on Fedora 32, 33, 34, and Rawhide.  Results are
comparable on Fedora 32 and 33.  On Fedora 34 and Rawhide, with this
commit in place, testing completes when the target_board is
native-extended-gdbserver.  On those OSes, when not using this commit,
testing usually hangs due to a problem with
gdb.threads/fork-plus-threads.exp.  I've also tested on all of the
mentioned OSes with target_board=native-gdbserver; for that testing,
I achieved comparable results over a number of runs.  (Unfortunately
results are rarely identical due to racy tests.)

gdb/testsuite/ChangeLog:

	* lib/gdbserver-support.exp (gdbserver_exit): Use the
	"-nowait" flag when waiting for gdbserver to exit.