Only run popcount native test on x86_64.
Only run popcount native test on x86_64.

dsroche/flint2,fredrik-johansson/flint2,dsroche/flint2,wbhart/flint2,fredrik-johansson/flint2,jpflori/flint2,fredrik-johansson/flint2,wbhart/flint2,jpflori/flint2,jpflori/flint2,jpflori/flint2,wbhart/flint2,dsroche/flint2,dsroche/flint2