backport -D flags to potion and more p2 enhancements

potion:
    -Llibdir
    -DitpvGJ debug flags
    --check
    -DG GC_CHECK

enable -fno-omit-frame-pointer
better potion_dump_stack
skip asan checks for the GC stack walkers
p2: parse ifexpr statements (not yet)
fix gc-test segv: skip empty or invalid PN_VTABLE(ptr->vt)
debug print to STDERR, better gc-test check

also debug print GC marked objects in the GC areas