[CVS ci] GC handling code review

[Leopold Toetsch]

This patch fixed several problems WRT GC:

- parrot -t examples/assembly/mandel.pasm 2> /dev/null took several
  100 MByte to run because:
- DOD/GC was disabled in Parrot_vsprintf and friends
- removing unnecessary DOD/GC block statements revealed some bugs:
- the method_table inside VTABLEs was not properly marked and
- t/src/*.t have to set interpreter->lo_var_ptr now, because
  they never enter a run-loop, where the stack limit is set.
- finally hash.c WRT GC is straightend and fixed
leo