Z180 support

Description

This PR establishes support for a Zilog Z180 port, using the Programmable Reload Timer 1, configured at 256 Hz.

Because of the generality of the Z180, the address of the Interrupt Vector for the programmable timer PRT1 is configurable, and must be configured by the crt0.asm outside of this port. A configuration assumption has been made, which should be checked against the actual system environment.

The two compilers (used by the z88dk) are supported. The sccz80 compiler and the sdcc compiler. The in-line assembly language notation used can be read by both compilers.

Background

This PR is based on running code for the SC130/SC131 and YAZ180 platforms, and is maintained by the z88dk team in this z88dk-libraries repository.