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>