[AArch64][Windows] Add llvm-readobj support for save_any_reg unwind opcode.

This is primarily used for Arm64EC, but it could be used for other
non-standard calling conventions. The testcase is based on an Arm64EC
thunk generated by MSVC.

The name save_any_reg comes from Microsoft documentation, but the full
encoding isn't specified there; this is reverse-engineered from the
behavior of the unwinder. (Thanks to Martin Storsjö for his example of
how to write simple unwinder testcases by directly calling
RtlVirtualUnwind.)

Differential Revision: https://reviews.llvm.org/D135196