[ARM64EC 3/?] Mark reserved registers specific to ARM64EC ABI.

Part of patchset to add initial support for ARM64EC.

I'm not completely sure I understand the reason for this restriction,
but Microsoft documentation says that asynchronous signals clobber these
registers, so we can't ever use them.

As far as I know, none of these registers have any hardcoded meaning, so
reserving them shouldn't have any significant side-effects.

Differental Revision: https://reviews.llvm.org/D125413