#997 r6k port for Rabbit 6000

[Philipp Klaus Krause]

We are already working on r4k and r5k ports for better Rabbit 4000 and Rabbit 5000 support. WE might as well create a Rabbit 6000 port to make use of the new instructions in the Rabbit 6000.
There is a separate ticket for Rabbit 6000 uCsim support, which we will need to be able to te4st this port: [feature-requests:#994].

[Philipp Klaus Krause]

The port exists for quite a while now; it uses R6K-specific instructions to generate better code than R4K/R5K, but there has been no testing on real hardware so far, and the r6k port is still considered experimental.
Complications: AFAIK, all my Rabbit 6000 hardware (RCM) uses 16-bit memories, SDCC assumes 8-bit memories; OpenRabbit supports neither the Rabbit 6000, nor 16-bit memories.