package/vlc: use BR2_TOOLCHAIN_HAS_LIBATOMIC

While fixing
http://autobuild.buildroot.net/results/d95/d9572bcafc1776eebe88bbcaf031a05cd6ee4b23/
by using http://patchwork.ozlabs.org/patch/674337/

 the build stopped at
a later stage with

/home/buildroot/br4/output/build/vlc-2.2.4/src/.libs/libvlccore.so: undefined reference to `__atomic_fetch_add_4'
/home/buildroot/br4/output/build/vlc-2.2.4/src/.libs/libvlccore.so: undefined reference to `__atomic_fetch_sub_4'
/home/buildroot/br4/output/build/vlc-2.2.4/src/.libs/libvlccore.so: undefined reference to `__atomic_fetch_or_1'
/home/buildroot/br4/output/build/vlc-2.2.4/src/.libs/libvlccore.so: undefined reference to `__atomic_compare_exchange_4'
/home/buildroot/br4/output/build/vlc-2.2.4/src/.libs/libvlccore.so: undefined reference to `__atomic_exchange_1'
collect2: error: ld returned 1 exit status

so libatomic must be pulled in if necessary.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>