Use the target-lexicon crate.

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.

cranelift/Cargo.toml

@@ -31,6 +31,7 @@ serde_derive = "1.0.8"
 term = "0.5.1"
 capstone = "0.3.1"
 wabt = { version = "0.3", optional = true }
+target-lexicon = "0.0.0"
 
 [features]
 default = ["wasm"]