As I described in the email I just sent, ATTACH_NO_WAIT being a host
macro, is wrong to be applied to other targets other than the
native target.  Due to it, the "target extended-remote; attach" sequence
behaves differently depending on the host.

This patch replaces the macro by a target property.

The only hosts that define it currently are Windows hosts,
and GNU Hurd.  I've tested the patch on
x86_64-unknown-linux-gnu {native/gdbserver}
and x86-pc-cygwin {native/gdbserver}.  I don't have access
to a Hurd system, but the change seems pretty obvious to me.

OK?

-- 
Pedro Alves