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/riscv/binemit.rs

@@ -0,0 +1,52 @@
+//! Emitting binary RISC-V machine code.
+
+use binemit::{CodeSink, bad_encoding};
+use ir::{Function, Inst, InstructionData};
+
+include!(concat!(env!("OUT_DIR"), "/binemit-riscv.rs"));
+
+/// R-type instructions.
+///
+///   31     24  19  14     11 6
+///   funct7 rs2 rs1 funct3 rd opcode
+///       25  20  15     12  7      0
+///
+/// Encoding bits: `opcode[6:2] | (funct3 << 5) | (funct7 << 8)`.
+fn recipe_r<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS) {
+    if let InstructionData::Binary { args, .. } = func.dfg[inst] {
+        let bits = func.encodings[inst].bits();
+        let rs1 = func.locations[args[0]].unwrap_reg();
+        let rs2 = func.locations[args[1]].unwrap_reg();
+        let rd = func.locations[func.dfg.first_result(inst)].unwrap_reg();
+
+        // 0-6: opcode
+        let mut i = 0x3;
+        i |= (bits as u32 & 0x1f) << 2;
+        // 7-11: rd
+        i |= (rd as u32 & 0x1f) << 7;
+        // 12-14: funct3
+        i |= ((bits as u32 >> 5) & 0x7) << 12;
+        // 15-19: rs1
+        i |= (rs1 as u32 & 0x1f) << 15;
+        // 20-24: rs1
+        i |= (rs2 as u32 & 0x1f) << 20;
+        // 25-31: funct7
+        i |= ((bits as u32 >> 8) & 0x7f) << 25;
+
+        sink.put4(i);
+    } else {
+        panic!("Expected Binary format: {:?}", func.dfg[inst]);
+    }
+}
+
+fn recipe_rshamt<CS: CodeSink + ?Sized>(_func: &Function, _inst: Inst, _sink: &mut CS) {
+    unimplemented!()
+}
+
+fn recipe_i<CS: CodeSink + ?Sized>(_func: &Function, _inst: Inst, _sink: &mut CS) {
+    unimplemented!()
+}
+
+fn recipe_iret<CS: CodeSink + ?Sized>(_func: &Function, _inst: Inst, _sink: &mut CS) {
+    unimplemented!()
+}