Reland "[.build-id] Create //prebuilt:.build-id target"

This is a reland of 9cb8bbc5cc3ed3e8f72acfcb17387af40b9b6763

Original change's description:
> [.build-id] Create //prebuilt:.build-id target
> 
> For the glory of the grand unified prebuilt .build-id directory.
> 
> I tested that this builds and does the right thing both when empty and
> populated.
> 
> Bug: 41478
> 
> Change-Id: I04815984eb2802a26322156e7c5a8e0ffb9eb26b
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/377560
> Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
> Reviewed-by: Jake Ehrlich <jakehehrlich@google.com>
> Reviewed-by: Francois Rousseau <frousseau@google.com>

Bug: 41478
Change-Id: Ia7cad7ff351f44336c079ba83db3f9b3f9cc55e9
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/377782
Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
Reviewed-by: Joshua Seaton <joshuaseaton@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 852b1e6..4cbefb3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -335,6 +335,7 @@
   data_keys = [ "prebuilt_binaries" ]
   deps = [
     "//build/images:packages",
+    "//prebuilt:.build-id",
     "//sdk",
   ]
 }
diff --git a/prebuilt/BUILD.gn b/prebuilt/BUILD.gn
new file mode 100644
index 0000000..d52a677
--- /dev/null
+++ b/prebuilt/BUILD.gn
@@ -0,0 +1,10 @@
+# Copyright 2020 The Fuchsia Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/prebuilt_binaries.gni")
+
+prebuilt_binaries(".build-id") {
+  testonly = true
+  build_id_dir = ".build-id"
+}
\ No newline at end of file