Transition to the SDCC tool chain

[TG9541]

A recent version of the SDCC tool chain shall be used for building 8051-eForth code.

Rationale:

• AVCASE51 is commercial legacy software - dealing with "abandonware" is risky
• SDCC isn't perfect but free and it's maintained by an active community
• SDCC is GPL but this doesn't appear to be in conflict with the MIT license of this code.
• SDCC 8051 run-time library won't be needed (but that's possibly uncritical due to "GPL+LE")
• the uCsim simulator has been applied for testing to STM8 eForth - this approach may also work for 8051-eForth