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/simplejit/Cargo.toml

@@ -15,6 +15,7 @@ cretonne-native = { path = "../native", version = "0.8.0", default-features = fa
 region = "0.2.0"
 libc = { version = "0.2.40", default-features = false }
 errno = "0.2.3"
+target-lexicon = { version = "0.0.0", default-features = false }
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3", features = ["winbase", "memoryapi"] }