Accept linkers representation for ELF segments with zero on-disk length.

Accept linkers representation for ELF segments with zero on-disk length.

For such segments, GNU bfd linker writes knowingly incorrect value
into the the file offset field of the program header entry, with the
motivation that file should not be mapped for creation of this segment
at all.

Relax checks for the ELF structure validity when on-disk segment
length is zero, and explicitely set mapping length to zero for such
segments to avoid validating rounding arithmetic.

PR:	217610
Reported by:	Robert Clausecker <fuz@fuz.su>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week