From: Arun Sharma <arun.sharma@intel.com>

The current Linux implementation of shmat() insists on SHMLBA alignment even
when shmflg & SHM_RND == 0.  This is not consistent with the man pages and
the single UNIX spec, which require only a page-aligned address.

However, some architectures require a SHMLBA alignment for correctness in all
cases.  Such architectures use __ARCH_FORCE_SHMLBA.