[llvm-objcopy] Fix handling of zero-size segments in llvm-objcopy

Some ELF files produced by lld may have zero-size segment placeholders as shown
below. Since GNU_STACK Offset is 0, the current code makes it the lowest used
offset, and relocates all the segments over the ELF header. The resulting
binary is total garbage.

This change fixes how llvm-objcopy handles PT_PHDR properlly by treating ELF
headers and the program header table as segments to allow the layout algorithm
decide where those should go.

Author: vit9696

Differential Revision: https://reviews.llvm.org/D42872

llvm-svn: 325189