Generate binemit::emit_inst() functions.

Use the meta language encoding recipes to generate an emit_inst()
function for each ISA. The generated calls into recipe_*() functions
that must be implemented by hand.

Implement recipe_*() functions for the RISC-V recipes.

Add the TargetIsa::emit_inst() entry point which emits an instruction to
a CodeSink trait object.

lib/cretonne/src/isa/intel/binemit.rs

@@ -0,0 +1,6 @@
+//! Emitting binary Intel machine code.
+
+use binemit::{CodeSink, bad_encoding};
+use ir::{Function, Inst};
+
+include!(concat!(env!("OUT_DIR"), "/binemit-intel.rs"));