package/qt5base: link with -latomic when needed

Qt 5.8 needs atomic operations, which are provided in libatomic when it
exists, like on sparc with gcc >= 4.8.

Fixes:
    http://autobuild.buildroot.org/results/49b/49bc9345b9849c9c3c53ace290c534ff7bb98683/
    http://autobuild.buildroot.org/results/9f2/9f213406954be51dfcad76ebdce8b73850842180/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Cc: Peter Seiderer <ps.report@gmx.net>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>