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.

gdb/ChangeLog:

	* i386-cygwin-tdep.c: Rename to...
	* i386-windows-tdep.c: ... this.
	* Makefile.in (ALL_TARGET_OBS): Rename i386-cygwin-tdep.c to
	i386-windows-tdep.c.
	* configure.tgt: Likewise.