Add RISC-V encodings for spill and fill.

Add a Stack() class for specifying operand constraints for values on the
stack.

Add encoding recipes for RISC-V spill and fill instructions. Don't
implement the encoding recipe functions yet since we don't have the
stack slot layout yet.

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

@@ -333,3 +333,11 @@ fn recipe_ujcall<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut
         panic!("Expected Call format: {:?}", func.dfg[inst]);
     }
 }
+
+fn recipe_gpsp<CS: CodeSink + ?Sized>(_func: &Function, _inst: Inst, _sink: &mut CS) {
+    unimplemented!();
+}
+
+fn recipe_gpfi<CS: CodeSink + ?Sized>(_func: &Function, _inst: Inst, _sink: &mut CS) {
+    unimplemented!();
+}