Move various lib helpers into libs/

[pevik]

• Migrate testcases/kernel/mem/hugetlb/lib/hugetlb.c into libs/ Helper library testcases/kernel/mem/hugetlb/lib/hugetlb.c should be moved inin libs/ directory (into newly created libs/hugetlb/libhugetlb.c).

• testcases/kernel/lib/numa_cpuset.c should be probably part of libs/numa/tst_numa.c. Also testcases/kernel/lib/numa_helper.c (if needed once migrate_pages0[1-3].c get rewritten into the new API).

• It's a question what to do with testcases/kernel/lib/ksm_helper.c (single function), maybe keep as is.