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/fuzz/Cargo.toml

@@ -20,6 +20,9 @@ git = "https://github.com/rust-fuzz/libfuzzer-sys.git"
 [dependencies.cretonne-wasm]
 path = "../lib/wasm"
 
+[dependencies.target-lexicon]
+version = "0.0.0"
+
 # Prevent this from interfering with workspaces
 [workspace]
 members = ["."]