elflint: Recognize and check SHF_COMPRESSED section flag.

SHF_COMPRESSED is a valid section flag, it can optionally be on any special
section, but it cannot be used on NOBITS sections or together with SHF_ALLOC.
A section that has SHF_COMPRESSED set must have a valid Chdr.

Signed-off-by: Mark Wielaard <mjw@redhat.com>