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/codegen/src/legalizer/call.rs

@@ -28,11 +28,7 @@ pub fn expand_call(
         _ => panic!("Wanted call: {}", func.dfg.display_inst(inst, None)),
     };
 
-    let ptr_ty = if isa.flags().is_64bit() {
-        ir::types::I64
-    } else {
-        ir::types::I32
-    };
+    let ptr_ty = isa.pointer_type();
 
     let sig = func.dfg.ext_funcs[func_ref].signature;
 

lib/codegen/src/legalizer/mod.rs

@@ -283,11 +283,7 @@ pub fn expand_stack_check(
         ir::InstructionData::UnaryGlobalVar { global_var, .. } => global_var,
         _ => panic!("Want stack_check: {}", func.dfg.display_inst(inst, isa)),
     };
-    let ptr_ty = if isa.flags().is_64bit() {
-        ir::types::I64
-    } else {
-        ir::types::I32
-    };
+    let ptr_ty = isa.pointer_type();
 
     let mut pos = FuncCursor::new(func).at_inst(inst);
     pos.use_srcloc(inst);