Merge of r4691: changed default alignment to 16.

Merge of r4691: changed default alignment to 16.

This fixes alignment problems observerd on ARMs, and likely also needed
for MIPSes.  Unless we know alignment is not required just assume we
need 16, which appears to be safe default for all architectures.

See here for details:
http://mailman.nginx.org/pipermail/nginx/2012-June/034139.html