MINOR: compiler: add new alignment macros

MINOR: compiler: add new alignment macros

This commit adds ALWAYS_ALIGN(), MAYBE_ALIGN() and ATOMIC_ALIGN() to
be placed as delimitors inside structures to force alignment to a
given size. These depend on the architecture's capabilities so that
it is possible to always align, align only on archs not supporting
unaligned accesses at all, or only on those not supporting them for
atomic accesses (e.g. before a lock).

(cherry picked from commit 226ef26056d9e6151c9838e70a3e2f4db5068cd8)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>