Add stubs for Intel and ARM architectures.

The Intel ISA handles both 32-bit and 64-bit code.

ARM is split into separate arm32 and arm64 ISAs since the architectures
have little in common in instruction encodings and register files.

lib/cretonne/meta/isa/arm64/__init__.py

@@ -0,0 +1,12 @@
+"""
+ARM 64-bit Architecture
+-----------------------
+
+ARMv8 CPUs running the Aarch64 architecture.
+"""
+
+from __future__ import absolute_import
+from . import defs
+
+# Re-export the primary target ISA definition.
+ISA = defs.ISA.finish()