Add a TargetIsa::allocatable_registers() method.

This gives the target ISA a chance to reserve registers like the stack
pointer or hard-wired 0 registers like %x0 on RISC-V.

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

@@ -2,6 +2,7 @@
 
 use ir;
 use isa::RegClass;
+use regalloc::AllocatableSet;
 use settings as shared_settings;
 use super::registers::{GPR, FPR};
 
@@ -16,3 +17,8 @@ pub fn legalize_signature(_sig: &mut ir::Signature,
 pub fn regclass_for_abi_type(ty: ir::Type) -> RegClass {
     if ty.is_int() { GPR } else { FPR }
 }
+
+/// Get the set of allocatable registers for `func`.
+pub fn allocatable_registers(_func: &ir::Function) -> AllocatableSet {
+    unimplemented!()
+}