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/module/src/module.rs

@@ -318,11 +318,7 @@ where
 
     /// Return then pointer type for the current target.
     pub fn pointer_type(&self) -> ir::types::Type {
-        if self.backend.isa().flags().is_64bit() {
-            ir::types::I64
-        } else {
-            ir::types::I32
-        }
+        self.backend.isa().pointer_type()
     }
 
     /// Create a new `Context` initialized for use with this `Module`.