gdb/testsuite: gdb.mi/mi-nonstop-exit.exp: enable non-stop using GDBFLAGS

When running

    make check TESTS="gdb.mi/mi-nonstop-exit.exp" RUNTESTFLAGS="--target_board=native-extended-gdbserver"

We get:

    220^error,msg="Unexpected vCont reply in non-stop mode: T05swbreak:;06:60d5ffffff7f0000;07:d0d2ffffff7f0000;10:5b57fdf7ff7f0000;thread:p2a4eed.2a4eed;core    :4;"^M
    (gdb) ^M
    UNRESOLVED: gdb.mi/mi-nonstop-exit.exp: first run: unable to start target

This is because non-stop is enabled using "-gdb-set non-stop 1".  This
doesn't work with the native-extended-gdbserver board, because with that
board GDB connects to GDBserver as soon as it's started.  Non-stop needs
to be enabled before connecting.

The usual pattern to follow is to set non-stop on the command line, like
gdb.mi/mi-nonstop.exp does.  Change the non-stop MI tests to use that
pattern.

The results diff when running gdb.mi/*.exp is:

    -# of expected passes           2877
    +# of expected passes           2938
     # of unexpected failures       34
     # of expected failures         8
     # of known failures            13
     # of unresolved testcases      4
     # of unsupported tests         1
    -# of duplicate test names      34
    +# of duplicate test names      35

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-nonstop-exit.exp: Enable non-stop through GDBFLAGS.
	* gdb.mi/mi-ns-stale-regcache.exp: Likewise.
	* gdb.mi/mi-nsintrall.exp: Likewise.
	* gdb.mi/mi-nsmoribund.exp: Likewise.
	* gdb.mi/mi-nsthrexec.exp: Likewise.
	* gdb.mi/mi-watch-nonstop.exp: Likewise.

Change-Id: Ic2736bedea8d448eee8c2b534d26b2427f6b4d27