Expands the vector register allocator from 20 to 28 registers by
including v8-v15. The HostToGuestThunk now saves/restores full Q8-Q15
(128-bit) instead of just D8-D15, and the GuestToHostThunk adds Q8-Q15
to its save/restore set since AAPCS64 only preserves their lower 64
bits across C calls.
Reduces register pressure for VMX-heavy guest code at the cost of
slightly larger thunk stack frames.
02d2cb5cc4