Re: elf: Add GNU_PROPERTY_MEMORY_SEAL gnu property

Don't run tests on targets without required support.  Supply an
explicit -z nomemory-seal rather then relying on the harness default,
to lessen confusion for people looking at the test.  Don't use numeric
labels for the sake of hppa64*-hpux, and run the tests there.  Remove
incorrect comment about source editing.  Also, xfail rather than
notarget failing tests with a list of target triples so we check that
the list is correct.