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/fuzz_translate_module.rs

@@ -3,13 +3,16 @@
 extern crate libfuzzer_sys;
 extern crate binaryen;
 extern crate cretonne_wasm;
+#[macro_use]
+extern crate target_lexicon;
 use cretonne_wasm::{translate_module, DummyEnvironment};
+use std::str::FromStr;
 
 fuzz_target!(|data: &[u8]| {
     let binaryen_module = binaryen::tools::translate_to_fuzz_mvp(data);
 
     let wasm = binaryen_module.write();
 
-    let mut dummy_environ = DummyEnvironment::default();
+    let mut dummy_environ = DummyEnvironment::with_triple(triple!("x86_64"));
     translate_module(&wasm, &mut dummy_environ).unwrap();
 });