elfcopy: copy raw (untranslated) contents to binary output

Previously elfcopy used elf_getdata to obtain data from ELF sections
being copied to binary output, but elf_getdata returns data that has
been translated - that is, data is in host byte order. When the host and
target differ in endianness (e.g., converting a big-endian MIPS ELF
object to binary on an x86 host) this resulted in byte-swapped data in
certain sections such as .dynamic.

Instead use elf_rawdata to keep data in the original, target endianness.

Reported by: Hiroki Mori <yamori83@yahoo.co.jp>, Bill Yuan
MFC after: 1 week
Sponsored by: The FreeBSD Foundation