elfutils: Add provisions for marking XFAILS in ptests

elfutils: Add provisions for marking XFAILS in ptests

elfuitls testsuites do not pass on all kind of distro features e.g.
with musl there are failures and with ld-is-lld less numer but it shows
failures too. Make run-ptest treat these as known failures.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>