common: add macros to silence invalid offsetof warnings

common: add macros to silence invalid offsetof warnings

Implementations are not forced to support offsetof for non-trivial types
in C++. gcc and clang both appear to support its use, but we should move
away from that. unfortunately that is not easy since offsetof is used
all over the place to use intrusive data structures.

I am unsure of the proper fix for this at the moment.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6f2cedb0da235ff3756248f370add13d46438cec