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.