Add an Encoding meta-language class.

Start adding some RISC-V encodings too as a way of testing the ergonomics.

meta/target/riscv/defs.py

@@ -0,0 +1,14 @@
+"""
+RISC-V definitions.
+
+Commonly used definitions.
+"""
+
+from cretonne import Target, CPUMode
+import cretonne.base
+
+target = Target('riscv', [cretonne.base.instructions])
+
+# CPU modes for 32-bit and 64-bit operation.
+RV32 = CPUMode('RV32', target)
+RV64 = CPUMode('RV64', target)