gdb.mi/mi-info-os.exp: Fix cross-debugger testing

gdb.mi/mi-info-os.exp: Fix cross-debugger testing

A live target is required for `-info-os' to work in non-native
configurations.

 (gdb)
 Expecting: ^(-info-os[
 ]+)?(.*\^done,OSDataTable=.*[
 ]+[(]gdb[)]
 [ ]*)
 -info-os
 ^error,msg="Don't know how to get OS data.  Try \"help target\"."
 (gdb)
 FAIL: gdb.mi/mi-info-os.exp: -info-os

If GDB does have a native configuration included, but we're testing
remote, it'll be worse, as if we're not connected yet, -info-os will
run against the default run target, and pass, falsely giving the
impression the remote bits were exercised.

gdb/testsuite/
2014-01-09  Maciej W. Rozycki  <macro@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.mi/mi-info-os.exp: Connect to the target with
        mi_gdb_target_load.