Revert "Disable -Werror=attributes on gcc"

This reverts commit 7683df24.

Reason for revert: doesn't help with gcc, so removing

Original change's description:
> 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}

TBR=jkummerow@chromium.org,machenbach@chromium.org,mtrofin@chromium.org,gsathya@chromium.org,jochen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:6339

Change-Id: I581e4f0499ae0d7e3bc791fd6fa9988aabe64c5e
Reviewed-on: https://chromium-review.googlesource.com/494469Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45052}

BUILD.gn

@@ -466,13 +466,6 @@ 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.