Disable -Werror=attributes on gcc

The warning triggers even if the attributes don't change - it's enough
to declare them multiple times. Given that the other compilers don't
complain, just disable the warning on gcc for now.

R=jkummerow@chromium.org,mtrofin@chromium.org
BUG=v8:6339
NOTRY=true

Change-Id: Ie0fcc4feeb8568d4ab74ac65f6887523f3cdcbf9
Reviewed-on: https://chromium-review.googlesource.com/494106
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45045}

BUILD.gn

@@ -466,6 +466,13 @@ config("toolchain") {
     }
   }
 
+  # This really should be if (is_gcc) {}.
+  if (!is_clang && (is_linux || is_android)) {
+    # Suppress warnings about different multiple visibility annotations on
+    # symbols. See https://crbug.com/v8/6339.
+    cflags += [ "-Wno-attributes" ]
+  }
+
   if (is_win) {
     cflags += [
       "/wd4245",  # Conversion with signed/unsigned mismatch.