[llvm-cvtres] Add support for ARM64

Also change some default cases into llvm_unreachable in
WindowsResourceCOFFWriter, to make it easier to find if they
are triggerd from within e.g. lld, which supported ARM64 earlier
than llvm-cvtres did.

Differential Revision: https://reviews.llvm.org/D39892

llvm-svn: 317942