core: use memory fence on x86 too

Link: https://github.com/GeneAssembly/biosal/issues/744
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
documentation: update concepts table in readme

Link: https://github.com/GeneAssembly/biosal/issues/745
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>