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/defs.py

@@ -0,0 +1,11 @@
+"""
+ARM64 definitions.
+
+Commonly used definitions.
+"""
+from __future__ import absolute_import
+from cdsl.isa import TargetISA, CPUMode
+import base.instructions
+
+ISA = TargetISA('arm64', [base.instructions.GROUP])
+A64 = CPUMode('A64', ISA)