Documentation: Update documentation regarding --mem-usage' option

Earlier patch in this patchset, enables the '--mem-usage' makedumpfile
option on arm64 platforms.

In addition, I have validated the '--mem-usage' makedumpfile option
on several ppc64/ppc64le and s390x machines, so update the
'makedumpfile.8' documentation to indicate that '--mem-usage' option is
supported not only on x86_64, but also on ppc64, s390x and arm64
platforms.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>