[pushed] Restore some windows-tdep.c code

[Tom Tromey]

>>>>> "Christian" == Christian Biesinger <cbiesinger@google.com> writes:

>  Is it possible to have a windows-native gdb that isn't also using
>  windows-tdep?

Christian>  +      w32_prefix_command_valid = 1;

Christian> Seems like this could be made a bool.

Yeah.  I just wanted to revert this bit to as close as possible to the
earlier code.

I guess I was thinking this flag could be removed, but I see now that
both windows-nat and windows-tdep install variables here.  So, I guess
it's needed.

Tom