[llvm-objcopy]Add support for *-freebsd output formats

GNU objcopy can support output formats like elf32-i386-freebsd and
elf64-x86-64-freebsd. The only difference from their regular non-freebsd
counterparts that I have observed is that the freebsd versions set the
OS/ABI field to ELFOSABI_FREEBSD. This patch sets the OS/ABI field
according based on the format whenever --output-format is specified.

Reviewed by: rupprecht, grimar

Differential Revision: https://reviews.llvm.org/D59645

llvm-svn: 356737