CFI directives allow the context that was stored on the stack by
raise_func_trampoline to be used to unwind to any exception handlers as
required when dispatching an exception. However, as the dispatcher may change
its input context in e.g. BTCpuResetToConsistentState and these changes also
need to be used when unwinding, have the trampoline CFI directly refer to the
input context rather than a copy of it.