Merge pull request #5625 from liewegas/wip-memcpy

buffer: put inlined memcpy into a header; x86_64 only

Reviewed-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>
Reviewed-by: Steve Capper <steve.capper@linaro.org>
Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>

diff --cc src/common/buffer.cc
Simple merge