readelf: decode LA48 and ASG_DISABLE feature flags
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
(cherry picked from commit e79b51e2b2e35d6256a1ef5311ce02ec4032d21f)
(cherry picked from commit 5e9e7ad9a2a4bc95326df76c90b9244470fea72e)