[llvm-objcopy][ELF] --only-keep-debug: set offset/size of segments with no sections to zero

PR50160: we currently ignore non-PT_PHDR segments with no sections, not
accounting for its p_offset and p_filesz: this can cause an out-of-bounds write
in `writeSegmentData` if the p_offset+p_filesz is larger than the total file
size.

This can be fixed by setting p_offset=p_filesz=0. The logic nicely unifies with
the logic added in D90897.

Reviewed By: jhenderson, rupprecht

Differential Revision: https://reviews.llvm.org/D101560