756ae7a558
Reserves x19 to permanently hold the A64BackendContext pointer (x20 - sizeof(A64BackendContext)), set once in the HostToGuestThunk. Since x19 is AAPCS64 callee-saved, it survives all host calls for free. Eliminates repeated 2-instruction mov+sub sequences in stackpoint push/pop, stack synchronization, FPCR management, and reserved load/store paths. Reduces allocatable GPRs from 8 to 7 (like on x64).