Three predicates affect each extension:
- supports_m determines whether the target CPU supports the instruction set.
- enable_m determines if the instructions should be used, assuming they're
available.
- use_m is the predicate used to actually use the instructions.
Predcates are boolean functions. There will be ISA predicates and instruction
predicates.
The ISA predicates will be turned into member functions on the generated Flags
structs.
