Fix {mi-tracepoint-changed, mi-tsv-changed}.exp with native-extended-gdbserver

Fix {mi-tracepoint-changed, mi-tsv-changed}.exp with native-extended-gdbserver

Fixes:

 -FAIL: gdb.trace/mi-tracepoint-changed.exp: reconnect: break-info 1
 +PASS: gdb.trace/mi-tracepoint-changed.exp: reconnect: tracepoint created
 +PASS: gdb.trace/mi-tracepoint-changed.exp: reconnect: tracepoint on marker is installed
 +PASS: gdb.trace/mi-tracepoint-changed.exp: reconnect: break-info 1

 -FAIL: gdb.trace/mi-tsv-changed.exp: upload: tsv1 created
 -FAIL: gdb.trace/mi-tsv-changed.exp: upload: tsv2 created
 +PASS: gdb.trace/mi-tsv-changed.exp: upload: tsv1 created
 +PASS: gdb.trace/mi-tsv-changed.exp: upload: tsv2 created

These tests do something like this:

 #0 - start gdb/gdbserver normally
 #1 - setup some things in the debug session
 #2 - disconnect from gdbserver
 #3 - restart gdb
 #4 - reconnect to gdbserver

The problem is that the native-extended-gdbserver board always spawns
a new gdbserver instance in #3 (and has gdb connect to that).  So when
the test gets to #4, it connects to that new instance instead of the
old one:

 (gdb) spawn ../gdbserver/gdbserver --multi :2354
 Listening on port 2354
 target extended-remote localhost:2354
 Remote debugging using localhost:2354
 ...
 spawn ../gdbserver/gdbserver --multi :2355
 Listening on port 2355
 47-target-select extended-remote localhost:2355
 =tsv-created,name="trace_timestamp",initial="0"\n
 47^connected
 (gdb)
 ...
 47-target-select extended-remote localhost:2355
 47^connected
 (gdb)
 FAIL: gdb.trace/mi-tsv-changed.exp: upload: tsv1 created
 FAIL: gdb.trace/mi-tsv-changed.exp: upload: tsv2 created

testsuite/ChangeLog:
2015-04-16  Pedro Alves  <palves@redhat.com>

	* boards/native-extended-gdbserver.exp (mi_gdb_start): Don't start
	a new gdbserver if gdbserver_reconnect_p is set.