many tests restart gdbserver

[Aman Wardak]

When the testsuite is run in a cross environment, many of the tests
start up the gdbserver on the target twice and only use the second
instance.

 lib/gdb.exp:gdb_start() starts the first instance
 lib/gdb.exp:runto_main() starts the second

Is this actually done for a purpose (maybe its needed to do this
sometimes) ? Or can we optimize this?  Has anyone looked at it?

Thanks,
Aman