cutelyst: link with libatomic when needed

On some architectures, atomic binutils are provided by the libatomic
library from gcc. Linking with libatomic is therefore necessary,
otherwise the build fails with:

sparc-buildroot-linux-uclibc/sysroot/lib/libatomic.so.1: error adding symbols: DSO missing from command line

This is often for example the case on sparcv8 32 bit.

This atomic dependency is due to Qt >= 5.8, pkconfig can't be used as
Qt5 pc files does not mention this dependency

Fixes:
 - http://autobuild.buildroot.net/results/9e307ab9c7067b26d7b33a572204394808e25772



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>