Reftypes part two: add support for stackmaps.

This commit adds support for generating stackmaps at safepoints to the
new backend framework and to the AArch64 backend in particular. It has
been tested to work with SpiderMonkey.

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -1394,7 +1394,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
 
         Opcode::Trap | Opcode::ResumableTrap => {
             let trap_info = (ctx.srcloc(insn), inst_trapcode(ctx.data(insn)).unwrap());
-            ctx.emit(Inst::Udf { trap_info })
+            ctx.emit_safepoint(Inst::Udf { trap_info });
         }
 
         Opcode::Trapif | Opcode::Trapff => {
@@ -1432,10 +1432,11 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
                 trap_info,
                 kind: CondBrKind::Cond(cond),
             });
+            ctx.emit_safepoint(Inst::Udf { trap_info })
         }
 
         Opcode::Safepoint => {
-            panic!("safepoint support not implemented!");
+            panic!("safepoint instructions not used by new backend's safepoints!");
         }
 
         Opcode::Trapz | Opcode::Trapnz | Opcode::ResumableTrapnz => {