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/arm32/__init__.py

@@ -0,0 +1,13 @@
+"""
+ARM 32-bit Architecture
+----------------------
+
+This target ISA generates code for ARMv7 and ARMv8 CPUs in 32-bit mode
+(AArch32). We support both ARM and Thumb2 instruction encodings.
+"""
+
+from __future__ import absolute_import
+from . import defs
+
+# Re-export the primary target ISA definition.
+ISA = defs.ISA.finish()