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/lib.rs

@@ -41,7 +41,6 @@
     )
 )]
 #![no_std]
-#![cfg_attr(not(feature = "std"), feature(alloc))]
 
 #[cfg(not(feature = "std"))]
 #[macro_use]