[Bug d/108408] libphobos: Support building on *-*-cygwin

[ibuclaw at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108408

--- Comment #3 from ibuclaw at gcc dot gnu.org ---
So a big part of what's missing in druntime bindings is *any* declarations for
version(CRuntime_Newlib) targets.  Though as I understand it, Cygwin is a bit
of a hybrid in that matter.