Roll clang + fix warnings

Change-Id: Idad827db453fd72ac7c73a85a551c47fc88b8f4c
Bug: https://github.com/google/perfetto/issues/58
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index b7916b7..a2ce0af 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -47,6 +47,8 @@
       "-Wno-padded",
       "-Wno-reserved-id-macro",
       "-Wno-unknown-sanitizers",
+      "-Wno-unknown-warning-option",
+      "-Wno-poison-system-directories",
     ]
   }