generator: Allow generators to take arguments

generator: Allow generators to take arguments

Using some serious macro magic, this patch extends generators to allow
them to take arbitrary arguments.  The arguments are marshalled into a
structure placed at the far end of the generator's stack when it is
created.  Then, they're unmarshalled back into C parameters when we first
context switch into the generator.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>