Meta: Add the start of a gn build based on the LLVM gn build

"based on" in this context means "largely copied from"

Meta/gn/secondary/BUILD.gn

@@ -0,0 +1,11 @@
+import("//Meta/gn/build/toolchain/compiler.gni")
+
+group("default") {
+  deps = [ "//Userland/Libraries/LibCore" ]
+}
+
+# A pool called "console" in the root BUILD.gn is magic and represents ninja's
+# built-in console pool. (Requires a GN with `gn --version` >= 552353.)
+pool("console") {
+  depth = 1
+}