Ensure that we don't produce code for pre-ARMv7 without barriers
We are unable to produce load/store barriers for pre-ARMv7 targets.
Phab:D894 added dummy cases to SMP.h for these barriers to prevent the
build from failing under the assumption that there are no SMP-capable
devices of this vintage. However, Trac #10433 points out that it is more
correct to simply set NOSMP for such targets.
Tested By: rwbarton
Test Plan: Validate
Reviewers: erikd, rwbarton, austin
Reviewed By: rwbarton
Differential Revision: https://phabricator.haskell.org/D1704
GHC Trac Issues: Trac #10433