Use PrintDisassembler for --disassemble.

This required a clean-up at the same time, to fix some inconsistencies in the
way that the APIs and macros had been used.

The new code structure allows for a test-runner option to print the features
required by each instruction, but this is not implemented here.

Change-Id: I967c8bd44f6a832892fc854ca6643a5e6a682997