[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317942 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed