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

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+"""
+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()

lib/cretonne/meta/isa/arm32/defs.py

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+"""
+ARM 32-bit definitions.
+
+Commonly used definitions.
+"""
+from __future__ import absolute_import
+from cdsl.isa import TargetISA, CPUMode
+import base.instructions
+
+ISA = TargetISA('arm32', [base.instructions.GROUP])
+
+# CPU modes for 32-bit ARM and Thumb2.
+A32 = CPUMode('A32', ISA)
+T32 = CPUMode('T32', ISA)