package/ogre: link with libatomic when needed

package/ogre: 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:

/home/test/autobuild/run/instance-1/output-1/host/lib/gcc/xtensa-buildroot-linux-uclibc/8.3.0/../../../../xtensa-buildroot-linux-uclibc/bin/ld: ../../lib/libOgreMain.so.1.12.0: undefined reference to `__atomic_fetch_add_8'

This is often for example the case on sparc v8 32 bits.

Fixes:
 - http://autobuild.buildroot.org/results/3a09e2d1d26b19243244eb7f9235c85488a788d2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>