databases/mysql57-client: Fix build for i386
Make MySQL build-script assume that FreeBSD's Clang (on i386)
doesn't have "atomic_*" functions builtin implemented.
This is added to cmake/os/FreeBSD.cmake to make the merge for upstream easier.

This fixes the build of mysql57-client on the i386 again.

PR: 229605
Submitted by: PitDron <pitdron@gmail.com>
Reported by: gbromov@gmail.com
Sponsored by: The FreeBSD Foundation