Add basic Z80 CPU support

Add basic Z80 CPU support

Supported ISAs:
- Z80 (all undocumented instructions)
- Z180
- eZ80 (Z80 mode only)

Datasheets:
Z80: https://www.zilog.com/manage_directlink.php?filepath=docs/z80/um0080&extn=.pdf
Z180: https://www.zilog.com/manage_directlink.php?filepath=docs/z180/ps0140&extn=.pdf
eZ80: http://www.zilog.com/force_download.php?filepath=YUhSMGNEb3ZMM2QzZHk1NmFXeHZaeTVqYjIwdlpHOWpjeTlWVFRBd056Y3VjR1Jt

To debug Z80 programs using GDB you must configure and embed
z80-stub.c to your program (SDCC compiler is required). Or
you may use some simulator with GDB support.

gdb/ChangeLog:

	* Makefile.in (ALL_TARGET_OBS): Add z80-tdep.c.
	* NEWS: Mention z80 support.
	* configure.tgt: Handle z80*.
	* features/Makefile (XMLTOC): Add z80.xml.
	* features/z80-cpu.xml: New.
	* features/z80.c: Generate.
	* features/z80.xml: New.
	* z80-tdep.c: New file.
	* z80-tdep.h: New file.

gdb/stubs/ChangeLog:

	* z80-stub.c: New file.

Change-Id: Id0b7a6e210c3f93c6853c5e3031b7bcee47d0db9