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.
* initial set of work for windows fastcall (x64) call convention
- call conventions: rename `fastcall` to `windows_fastcall`
- add initial set of filetests
- ensure arguments are written after the shadow space/store (offset-wise)
The shadow space available before the arguments (range 0..32)
is not used as spill space yet.
* address review feedback
