[imcc] enable -O[12p] options again

Rename imcc_set_debug_api() to imcc_set_flags_api() to set
verbosity, debug flags and optimizer flags at once.

Remove unused internal imcc->IMCC_DEBUG flags.

-v prints now also the optimizer_level.

Document internal imcc_set_verbosity() as unused.

Makefile: add testOp, disable testexec (unused)

Add missing documentation to docs/running.pod and docs/binaries/parrot.pod
which is essentially the same. docs/running.pod should include more high-level
info. Remove gcdebug runcore documentation (unused).

Fix enabling parrot --imcc-debug flags.

Add Parrot_imcc_dflags and Parrot_imcc_opt_flags enums to include/imcc/api.h
for the frontends.

Mark t/op/calling.t test 63 as failing with -O1. (TODO)
This is a bad test, assuming an unoptimized layout.