Specialize amaranth code by platform

We would like to be able to specialize the hps_accel build when targeting the CrossLinkNX while still retaining the ability to work on other platforms. For now, we plan to pass in a boolean flag, but in the longer term it might be desirable to have a more generic mechanism.

Amaranth has a platform mechanism that looks perhaps suitable although:

(a) the mechanism is primarily concerned with IO and specializing for particular boards,
(b) we are currently generating verilog through a mechanism that sets platform to None.