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/machinst/compile.rs

@@ -23,7 +23,7 @@ where
     // Build the lowering context.
     let lower = Lower::new(f, abi, block_order)?;
     // Lower the IR.
-    let mut vcode = lower.lower(b)?;
+    let (mut vcode, stackmap_request_info) = lower.lower(b)?;
 
     debug!(
         "vcode from lowering: \n{}",
@@ -57,11 +57,23 @@ where
         }
     }
 
+    // If either there are no reference-typed values, or else there are
+    // but there are no safepoints at which we need to know about them,
+    // then we don't need stackmaps.
+    let sri = if stackmap_request_info.reftyped_vregs.len() > 0
+        && stackmap_request_info.safepoint_insns.len() > 0
+    {
+        Some(&stackmap_request_info)
+    } else {
+        None
+    };
+
     let result = {
         let _tt = timing::regalloc();
         allocate_registers_with_opts(
             &mut vcode,
             b.reg_universe(),
+            sri,
             Options {
                 run_checker,
                 algorithm,