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/enc_tables.rs

@@ -0,0 +1,8 @@
+//! Encoding tables for Intel ISAs.
+
+use ir::InstructionData;
+use ir::instructions::InstructionFormat;
+use ir::types;
+use isa::enc_tables::{Level1Entry, Level2Entry};
+
+include!(concat!(env!("OUT_DIR"), "/encoding-intel.rs"));