c/openmp_matrix: link with libatomic
c/openmp_matrix: link with libatomic

Building with clang -m32 on Sabayon fails with:

    undefined reference to `__atomic_load'

Add -latomic to fix this error.

fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared