Fix collection of objects referenced only from GC_mark_stack_X variables
(Apply commit a7a5666 from 'release-7_2' branch.)

* include/private/gc_pmark.h (mse): Move to gc_priv.h (as used by
GC_mark_stack_limit, GC_mark_stack_top, GC_mark_stack).
* include/private/gc_pmark.h (GC_mark_stack_limit, GC_mark_stack_top,
GC_mark_stack): Remove variable declaration.
* include/private/gc_priv.h (_GC_arrays): Add _mark_stack,
_mark_stack_limit, _mark_stack_top fields.
* include/private/gc_priv.h (GC_mark_stack_limit, GC_mark_stack_top,
GC_mark_stack): Define macro (redirecting to the corresponding field
of GC_arrays).
* mark.c (GC_mark_stack, GC_mark_stack_limit, GC_mark_stack_top):
Remove variable; move comment to gc_priv.h.
* mark.c (GC_push_marked1, GC_push_marked2, GC_push_marked4): Undefine
GC_mark_stack_top and GC_mark_stack_limit macros (before redefining
them to local variables) at function start, redefine them back to the
corresponding field of GC_arrays at function exit.

Conflicts:

    include/private/gc_pmark.h