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/filetests/filetests/vcode/aarch64/reftypes.clif

@@ -63,3 +63,65 @@ block0:
 ; nextln: mov sp, fp
 ; nextln: ldp fp, lr, [sp], #16
 ; nextln: ret
+
+function %f5(r64, r64) -> r64, r64, r64 {
+    fn0 = %f(r64) -> b1
+    ss0 = explicit_slot 8
+
+block0(v0: r64, v1: r64):
+    v2 = call fn0(v0)
+    stack_store.r64 v0, ss0
+    brz v2, block1(v1, v0)
+    jump block2(v0, v1)
+
+block1(v3: r64, v4: r64):
+    jump block3(v3, v4)
+
+block2(v5: r64, v6: r64):
+    jump block3(v5, v6)
+
+block3(v7: r64, v8: r64):
+    v9 = stack_load.r64 ss0
+    return v7, v8, v9
+}
+
+; check: Block 0:
+; check: stp fp, lr, [sp, #-16]!
+; nextln: mov fp, sp
+; nextln: sub sp, sp, #32
+; nextln: stp x19, x20, [sp, #-16]!
+; nextln: virtual_sp_offset_adjust 16
+; nextln: mov x19, x0
+; nextln: mov x20, x1
+; nextln: mov x0, x19
+; nextln: ldr x16, 8 ; b 12 ; data
+; nextln: stur x19, [sp, #24]
+; nextln: stur x20, [sp, #32]
+; nextln: (safepoint: slots [S0, S1]
+; nextln: blr x16
+; nextln: ldur x19, [sp, #24]
+; nextln: ldur x20, [sp, #32]
+; nextln: add x1, sp, #16
+; nextln: stur x19, [x1]
+; nextln: and w0, w0, #1
+; nextln: cbz x0, label1 ; b label3
+; check: Block 1:
+; check:  b label2
+; check: Block 2:
+; check: mov x0, x20
+; nextln: b label5
+; check: Block 3:
+; check: b label4
+; check: Block 4:
+; check: mov x0, x19
+; nextln: mov x19, x20
+; nextln: b label5
+; check: Block 5:
+; check: add x1, sp, #16
+; nextln: ldur x1, [x1]
+; nextln: mov x2, x1
+; nextln: mov x1, x19
+; nextln: ldp x19, x20, [sp], #16
+; nextln: mov sp, fp
+; nextln: ldp fp, lr, [sp], #16
+; nextln: ret