x86-64: fix ZMM register state tracking

x86-64: fix ZMM register state tracking

The three AVX512 state components are entirely independent - one being
in its "init state" has no implication whatsoever on either of the other
two. Fully separate X86_XSTATE_ZMM_H and X86_XSTATE_ZMM handling, to
prevent upper halves of the upper 16 ZMM registers to display as if they
were zero (when they aren't) after e.g. VZEROALL/VZEROUPPER.