WONTFIX empty registers -1 from :immediate #1024

We cannot fix :immediate for such all cases, as the globals
are not fixed up within compile-time :immediate calls by definition,
they are even cleared to be fixed up by the immediate call.
See http://trac.parrot.org/parrot/ticket/1324
Disable t/compilers/imcc/syn/const.t test 41

But add --debugging assertions for most cases: set_p_pc, set_p_p.
Also throw on NULL PMC arguments for CallContext (sub args)
with "CallContext: Empty PMC argument"

Closes GH #1024