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.
To keep cross-compiling straightforward, Cretonne shouldn't have any
behavior that depends on the host. This renames the "Native" calling
convention to "SystemV", which has a defined meaning for each target,
so that it's clear that the calling convention doesn't change
depending on what host Cretonne is running on.
The spiller wasn't tracking register pressure correctly for dead EBB
parameters in visit_ebb_header(). Make sure we free any dead EBB
parameters.
Fixes#223
