[patch cygwin]: Increase stack limit to 8MB as for mingw targets

[Paolo Bonzini]

> ??  GCC requires GNU make for quite some time.  install.texi says:
> 
> @item GNU make version 3.80 (or later)
> 
> You must have GNU make installed to build GCC@.
> 
> and we use many GNU make extensions.

Yeah, but winsup (from the src tree) does not require GNU make, and
mh-mingw is shared between the two.  It was decided that the toplevel
and config/mt-* should not use GNU make extensions, only config/mh-*.

Paolo