Optimize use of uc_context_save/restore

[andreiw]

Technically these should only be needed if the previous context was interrupted. If the last context was in a native call, we should be able to get away with saving/restoring minimal state as per calling convention (RBX, RBP, RDI, RSI, RSP, R12, R13, R14, R15, and XMM6-XMM15)

Seems like a fairly minor optimization, as realistically there won't be giant chains of emu->native->emu invocations, while event handling would still involve a full context save (akin to an interrupt handler, really).