[llvm-objcopy] Change -O binary to respect section removal and behave like GNU objcopy

The original -O binary implementation just copied segment data from the
object and dumped it into a file. This doesn't take into account any
operations performed on objects such as section removal. GNU objcopy has
some specific behavior that we'd also like to respect. For instance
using -O binary and -j <some_section> will dump <some_section> to a
file. This change implements GNU objcopy style -O binary to as close of
an approximation as I can determine.

Differential Revision: https://reviews.llvm.org/D39713

llvm-svn: 318324