macros: add bson_sync_synchronize() macro for basic memory barrier.
macros: add bson_sync_synchronize() macro for basic memory barrier.

We might actually want to do a per-architecture version of this so that
we can do a full fence. But this should work for now.

ajdavis/libbson,mapr/libbson,paulmelnikow/libbson,paulmelnikow/libbson,mongodb/libbson,mongodbinc-interns/libbson,rayyang2000/libbson,mongodb/libbson,jqk6/libbson,mapr/libbson,remicollet/libbson,4Second2None/libbson_tmp,4Second2None/libbson_tmp,paulmelnikow/libbson,remicollet/libbson,paulmelnikow/libbson,rayyang2000/libbson,mongodbinc-interns/libbson,remicollet/libbson,ksuarz/libbson,bjori/libbson,remicollet/libbson,ksuarz/libbson,mongodbinc-interns/libbson,bjori/libbson,Convey-Compliance/libbson,rcsanchez97/libbson,ksuarz/libbson,jqk6/libbson,Convey-Compliance/libbson,alexeyvo/libbson,Convey-Compliance/libbson,bjori/libbson,Machyne/libbson,Machyne/libbson,jqk6/libbson,jqk6/libbson,Machyne/libbson,derickr/libbson,4Second2None/libbson_tmp,rayyang2000/libbson,rcsanchez97/libbson,mongodbinc-interns/libbson,Machyne/libbson,ajdavis/libbson,alexeyvo/libbson,alexeyvo/libbson,derickr/libbson,rcsanchez97/libbson,mongodb/libbson,rayyang2000/libbson,derickr/libbson,derickr/libbson,paulmelnikow/libbson,ajdavis/libbson,4Second2None/libbson_tmp,mapr/libbson