This switches from a custom list of architectures to use the
target-lexicon crate.
- "set is_64bit=1; isa x86" is replaced with "target x86_64", and
similar for other architectures, and the `is_64bit` flag is removed
entirely.
- The `is_compressed` flag is removed too; it's no longer being used to
control REX prefixes on x86-64, ARM and Thumb are separate
architectures in target-lexicon, and we can figure out how to
select RISC-V compressed encodings when we're ready.
* Optimize 0.0 floating point constants. Rather than using the existing
process of emitting bit patterns and moving them into floating point
registers, use the `xorps` instruction to zero out the register.
* is_zero predicate function will not accept negative zero. Fixed formatting for encoding recipe and filetests.
