qt5base: make harfbuzz support selectable

If selected use:

 - system/buildroot harfbuzz in case __sync for 4 bytes is supported
 - qt harfbuzz otherwise (using QAtomic instead)

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>