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/meta/gen_encoding.py

@@ -91,6 +91,8 @@ def emit_instps(instps, fmt):
     Emit a function for matching instruction predicates.
     """
 
+    if not instps:
+        fmt.line('#[allow(unused_variables)]')
     with fmt.indented(
             'pub fn check_instp(inst: &InstructionData, instp_idx: u16) ' +
             '-> bool {', '}'):