unwind-d2 build workaround for arm64

The __builtin_init_dwarf_reg_size_table function is unimplemented in
clang 3.6 for AArch64. Comment it out for now and replace it with
a message and abort.

Tracked in upstream LLVM PR 22997
https://llvm.org/bugs/show_bug.cgi?id=22997

Submitted by: andrew