Make the .eh_frame_hdr code work on FreeBSD as well.

We currently only include <link.h> on CloudABI and Linux. We can enable
it on FreeBSD as well, as it also supports the dl_iterate_phdr()
function that's provided by <link.h>.

FreeBSD, however, does not provide the ElfW() macro. Instead, the
host-specific ELF datastructures are named just Elf_XXX in addition to
the host-independent Elf32_XXX and Elf64_XXX types.

Differential Revision:	http://reviews.llvm.org/D8169
Approved by:	emaste

llvm-svn: 236154