package/leveldb: link with libatomic when needed

package/leveldb: link with libatomic when needed

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

/home/buildroot/autobuild/instance-2/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/7.4.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: libleveldb.so.1.22.0: undefined reference to `__atomic_fetch_sub_4'

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

Fixes:
 - http://autobuild.buildroot.net/results/257a820bb87a874ce0e2152d2752845be972fabe

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>