[objcopy] make objcopy follow program header standards

Submitted on behalf of Armando Montanez (amontanez@google.com).

Objects with unused program headers copied by objcopy would always have
nonzero values for program header offset and program header entry size.
While technically valid, this atypical behavior triggers warnings in some
tools. This change sets the two fields to zero when the program header is
unused, better fitting the general expectations for unused program header
data.

Section headers behaved somewhat similarly (though only with the entry size),
and are fixed in this revision as well.

Differential Revision: https://reviews.llvm.org/D51961

llvm-svn: 342065