platform2: move --std=gnu++11 flag to common-mk/common.gypi

A number of platform2 projects enabled C++11 features individually
in their .GYP files. Removed this flag from each of the projects
and added it to common.gypi for everyone to share.

BUG=None
TEST=Compile various platform2 targets to make sure they still compile.

Change-Id: I4136573ec5dcc988df2826056c8dae8847725504
Reviewed-on: https://chromium-review.googlesource.com/211534
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>

buffet/buffet.gyp

diff --git a/buffet/buffet.gyp b/buffet/buffet.gyp
index 6254e56..820b25c 100644
--- a/buffet/buffet.gyp
+++ b/buffet/buffet.gyp
@@ -15,9 +15,6 @@
         '-lbase-dbus_test_support-<(libbase_ver)',
       ],
     },
-    'cflags_cc': [
-      '-std=gnu++11',
-    ],
   },
   'targets': [
     {