gdb: rename i386-cygwin-tdep.c to i386-windows-tdep.c
Since this file contains things that apply not only to Cygwin binaries,
but also to non-Cygwin Windows binaries, I think it would make more
sense for it to be called i386-windows-tdep.c. It is analogous to
amd64-windows-tdep.c, which we already have.