Rename meta/target -> meta/isa.

Clarify terminology by always referring to a 'Target ISA' instead of just
'Target'. Use 'isa' as a module name instead of 'target' both in Rust and Python
code.

This is only to clarify terminology and not at all because Cargo insists on
using the 'target' sub-directory for build products. Oh, no. Not at all.

meta/isa/riscv/defs.py

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