Work around non-existence of ElfW(type) macro on FreeBSD

Fixes PR51331. On FreeBSD, the elf headers don't (yet) provide the
ElfW(type) macro. However, there is a similar set of macros in the
<sys/elf-generic.h> header, of which `__ElfN(type)` exactly matches the
indended purpose.

Reviewed By: gulfem

Differential Revision: https://reviews.llvm.org/D107388