On visual c 2013 and earlier a warning is generated if externs
are not declared with the same alignment as the declaration, when
using /ltcg

BUG=libyuv:633
TEST=standalong test built with cl /Bv /GL /Ox /nologo a.cc b.cc /link /ltcg
R=skal@google.com

Review URL: https://codereview.chromium.org/2291533004 .