[vm/build] Ensure relevant artifacts are built for precompiled target

Include gen_snapshot and platform dill targets to runtime_precompiled to ensure this target can be used separately without runtime target when working on AOT tests.
Avoiding runtime target saves time building unneccessary snapshots.

Change-Id: Ib6583772f47af1eecaf9a9849b3966461982502f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/365870
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

BUILD.gn

@@ -86,7 +86,10 @@ group("run_ffi_unit_tests") {
 group("runtime_precompiled") {
   deps = [
     "runtime/bin:dart_precompiled_runtime",
+    "runtime/bin:gen_snapshot",
+    "runtime/bin:gen_snapshot($host_toolchain)",
     "runtime/bin:process_test",
+    "runtime/vm:kernel_platform_files($host_toolchain)",
   ]
   if (is_linux || is_android) {
     deps += [ "runtime/bin:abstract_socket_test" ]