gdb: rename content of i386-windows-tdep.c, cygwin to windows
i386-cygwin-tdep.c has just been renamed to i386-windows-tdep.c, this
patch now renames everything in it that is not Cygwin-specific to
replace "cygwin" with "windows".
Note that I did not rename i386_cygwin_core_osabi_sniffer, since that
appears to be Cygwin-specific.
gdb/ChangeLog:
* i386-windows-tdep.c: Mass-rename "cygwin" to "windows", except
i386_cygwin_core_osabi_sniffer.