[lld] Use correct machine type in ARM64EC COFF headers.

This adds very minimal support for ARM64EC/ARM64X targets,
just enough for interesting test cases. Next patches in the
series extend llvm-objdump and llvm-readobj to provide
better tests. Those will also be useful for testing further
ARM64EC LLD support.

Differential Revision: https://reviews.llvm.org/D149086