unix: use vDSO for getrandom() on linux

With CL 614835 adding support in the runtime for calling into the
getrandom() vDSO function, wire up x/sys/unix's Getrandom() function to
it, so that callers can benefit from the increased speed and shared
vDSO state with the runtime.

Updates golang/go#69577.

Change-Id: I17734409982c51bb984a6337f4ffa8f60414ebee
Reviewed-on: https://go-review.googlesource.com/c/sys/+/615335
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

unix/vgetrandom_linux.go

@@ -0,0 +1,12 @@
+// Copyright 2024 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.
+
+//go:build linux && go1.24
+
+package unix
+
+import _ "unsafe"
+
+//go:linkname vgetrandom runtime.vgetrandom
+func vgetrandom(p []byte, flags uint32) (ret int, supported bool)