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/base/settings.py

@@ -20,6 +20,8 @@ opt_level = EnumSetting(
 
 is_64bit = BoolSetting("Enable 64-bit code generation")
 
+is_compressed = BoolSetting("Enable compressed instructions")
+
 enable_float = BoolSetting(
         """Enable the use of floating-point instructions""",
         default=True)