[VM] Make runtime-kernel the default build target. This ensures that kernel_services snapshot is built and available.

Change-Id: I023529fe11f3ad5d199faff409a43f83e51cced2
Reviewed-on: https://dart-review.googlesource.com/54634
Reviewed-by: Zach Anderson <zra@google.com>

BUILD.gn

@@ -13,7 +13,7 @@ group("default") {
     testonly = true
   }
   deps = [
-    ":runtime",
+    ":runtime_kernel",
   ]
 }