signal: Move addr_lsb into the _sigfault union for clarity

The addr_lsb fields is only valid and available when the
signal is SIGBUS and the si_code is BUS_MCEERR_AR or BUS_MCEERR_AO.
Document this with a comment and place the field in the _sigfault union
to make this clear.

All of the fields stay in the same physical location so both the old
and new definitions of struct siginfo will continue to work.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>