pthread version of bson_context_get_oid_seq64_threadsafe and bson_context_get_oid_seq32_threadsafe

pthread version of bson_context_get_oid_seq64_threadsafe and bson_context_get_oid_seq32_threadsafe

With this patch, libbson builds under Raspbian on RaspberryPi (which has no __sync_fetch_and_add_8) and pass all tests.