Fill in boilerplate for Intel and ARM targets.

The intel, arm32, and arm32 targets were only defined in the meta
language previously. Add Rust implementations too.

This is mostly boilerplate, except for the unit tests in the
registers.rs files.

lib/cretonne/src/isa/intel/settings.rs

@@ -0,0 +1,9 @@
+//! Intel Settings.
+
+use settings::{self, detail, Builder};
+use std::fmt;
+
+// Include code generated by `lib/cretonne/meta/gen_settings.py`. This file contains a public
+// `Flags` struct with an impl for all of the settings defined in
+// `lib/cretonne/meta/cretonne/settings.py`.
+include!(concat!(env!("OUT_DIR"), "/settings-intel.rs"));