runtime: fix syscall9 on darwin/arm64

The aarch64 ABI says that only the first 8 arguments should be
passed as registers, subsequent  arguments should be put on
the stack.

Syscall9 is not putting the 9th argument on the stack, and it should.

The standard library hasn't hit this issue because it uses Syscall9
for functions that only require 7 or 8 parameters.

Change-Id: I1fafca5b16f977ea856e3f08b4ff3d0a2a7a4dfe
Reviewed-on: https://go-review.googlesource.com/c/go/+/702297
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/syscall_test.go

@@ -0,0 +1,28 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"internal/testenv"
+	"runtime"
+	"testing"
+)
+
+func TestSyscallArgs(t *testing.T) {
+	if runtime.GOOS != "darwin" {
+		t.Skipf("skipping test: GOARCH=%s", runtime.GOARCH)
+	}
+	testenv.MustHaveCGO(t)
+
+	exe, err := buildTestProg(t, "testsyscall")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, exe)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("test program failed: %v\n%s", err, out)
+	}
+}