Windows gdb: Enable "set scheduler-locking on"

Windows gdb: Enable "set scheduler-locking on"

Surprisingly (to me), enabling scheduler locking on Windows currently
fails:

 (gdb)
 set scheduler-locking on
 Target 'native' cannot support this command.

The backend itself does support scheduler-locking.  This patch
implements windows_nat_target::get_thread_control_capabilities so that
the core knows schedlocking works for this target.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Ie762d3768fd70e4ac398c8bcc03c3213bfa26a6a