backport -D flags and more p2 enhancements

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

enable -fno-omit-frame-pointer
compile only one vm target into the executable, no cross-compiler support.
add potion_dump helper with -DDEBUG
better potion_dump_stack
add runtime tracing with --bytecode
add parser tracing with -Dp (not yet)
backport from p2: no asan checks for stack walker

decr the stack is asan unsafe
fix -Dvp: set YY_DEBUG on DEBUG
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