[gc] add missing PackFileView to continuation.mark

also simplify the code a bit
[gc] protect pcc_reuse_continuation from empty data

check for uninitialized continuation->data and allocate fresh then also.
simplify code a bit.
[gc] check empty work_list, optimize gc_gms for -UMEMORY_DEBUG

do not call dummy functions in the common case during GC.

for -DMEMORY_DEBUG only:
protect from empty work_list in check_sanity, which can
occur after the GC was already unblocked.