upgrade to target-lexicon 0.8.0

* the target-lexicon crate no longer has or needs the std feature
  in cargo, so we can delete all default-features=false, any mentions
  of its std feature, and the nostd configs in many lib.rs files
* the representation of arm architectures has changed, so some case
  statements needed refactoring

cranelift/codegen/src/isa/call_conv.rs

@@ -31,6 +31,7 @@ impl CallConv {
             // uses System V.
             Ok(CallingConvention::SystemV) | Err(()) => CallConv::SystemV,
             Ok(CallingConvention::WindowsFastcall) => CallConv::WindowsFastcall,
+            Ok(unimp) => unimplemented!("calling convention: {:?}", unimp),
         }
     }