includes/stg/SMP.h: implement simple load_/store_load_barrier on armv6 and older

Assuming there is no real SMP systems on these CPUs
I've added only compiler barrier (otherwise write_barrier
and friends need to be fixed as well).

Patch also fixes build breakage reported in Trac #10244.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Reviewers: rwbarton, nomeata, austin

Reviewed By: nomeata, austin

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D894

GHC Trac Issues: Trac #10244