Implement the machinery to create a TargetIsa.

Add an isa::lookup() function which serves as a target registry for creating
Box<TargetIsa> trait objects.

An isa::Builder makes it possible to confugure the trait object before it is
created.

src/libcretonne/isa/riscv/mod.rs

@@ -1,3 +1,37 @@
 //! RISC-V Instruction Set Architecture.
 
 pub mod settings;
+
+use super::super::settings as shared_settings;
+use super::Builder as IsaBuilder;
+use super::{TargetIsa, Encoding};
+use ir::dfg::DataFlowGraph;
+use ir::entities::Inst;
+
+#[allow(dead_code)]
+struct Isa {
+    shared_flags: shared_settings::Flags,
+    isa_flags: settings::Flags,
+}
+
+pub fn isa_builder() -> IsaBuilder {
+    IsaBuilder {
+        setup: settings::builder(),
+        constructor: isa_constructor,
+    }
+}
+
+fn isa_constructor(shared_flags: shared_settings::Flags,
+                   builder: shared_settings::Builder)
+                   -> Box<TargetIsa> {
+    Box::new(Isa {
+        shared_flags: shared_flags,
+        isa_flags: settings::Flags::new(builder),
+    })
+}
+
+impl TargetIsa for Isa {
+    fn encode(&self, _: &DataFlowGraph, _: &Inst) -> Option<Encoding> {
+        unimplemented!()
+    }
+}