[ARM64EC 1/?] Add parsing support to llvm-objdump/llvm-readobj.

This is the first patch of a patchset to add initial support for
ARM64EC. Basic documentation is available at
https://docs.microsoft.com/en-us/windows/uwp/porting/arm64ec-abi .
(Discourse post:
https://discourse.llvm.org/t/initial-patches-for-arm64ec-windows-11-now-posted/62449
.)

The file format for ARM64EC is basically identical to normal ARM64.
There are a few extra sections, but the existing code for reading ARM64
object files just works.

Differential Revision: https://reviews.llvm.org/D125411