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.

lib/reader/Cargo.toml

@@ -10,6 +10,7 @@ readme = "README.md"
 
 [dependencies]
 cretonne-codegen = { path = "../codegen", version = "0.8.0" }
+target-lexicon = { version = "0.0.0" }
 
 [badges]
 maintenance = { status = "experimental" }