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/Userland/Libraries/LibSystem/BUILD.gn

@@ -0,0 +1,32 @@
+group("LibSystem") {
+  deps = []
+  if (current_os == "serenity") {
+    deps += [
+      ":system_shared",
+      ":system_static",
+    ]
+  }
+}
+
+if (current_os == "serenity") {
+  shared_library("system_shared") {
+    output_name = "system"
+    cflags_cc = [ "-nostdlib" ]
+    ldflags = [
+      "-nostdlib",
+      "-static-libstdc++",
+    ]
+    sources = [
+      "syscall.cpp",
+      "syscall.h",
+    ]
+  }
+
+  static_library("system_static") {
+    output_name = "system"
+    sources = [
+      "syscall.cpp",
+      "syscall.h",
+    ]
+  }
+}