[compiler-rt] Define ElfW() macro if it's not defined

Summary:
The `ElfW()` macro is not provided by `<link.h>` on some
systems (e.g., FreeBSD). On these systems the data structures are
just called `Elf_XXX`. Define `ElfW()` locally.

(This fix is taken from [libunwind](https://github.com/llvm/llvm-project/blob/9b05596eff21388ae6f17cdea8df0538dc630441/libunwind/src/AddressSpace.hpp#L144-L157).)

Reviewers: compnerd

Differential revision: https://reviews.llvm.org/D75907