[llvm-objcopy]Preserve data in segments not covered by sections

llvm-objcopy previously knew nothing about data in segments that wasn't
covered by section headers, meaning that it wrote zeroes instead of what
was there. As it is possible for this data to be useful to the loader,
this patch causes llvm-objcopy to start preserving this data. Data in
sections that are explicitly removed continues to be written as zeroes.

This fixes https://bugs.llvm.org/show_bug.cgi?id=41005.

Reviewed by: jakehehrlich, rupprecht

Differential Revision: https://reviews.llvm.org/D59483

llvm-svn: 356919