Add support for multiple large page sizes for UseSHM

If we decide not to remove the support for UseSHM (see JDK-8261894), we should add support for using different large page sizes. Currently shmget() is only called with SHM_HUGETLB and this will always use the default large page size. For systems supporting multiple page sizes we should make use of SHM_HUGE_2MB or SHM_HUGE_1GB to allow usage of other page sizes than the default.