Add public GC_set/get_abort_func to replace default GC_on_abort

Add public GC_set/get_abort_func to replace default GC_on_abort

* include/gc.h (GC_abort_func): New public typedef.
* include/gc.h (GC_set_abort_func, GC_get_abort_func): New API
function declaration.
* include/private/gc_priv.h (GC_on_abort): Change function
declaration to function pointer of GC_abort_func type (only if
SMALL_CONFIG and not PCR).
* misc.c (GC_on_abort): Rename to GC_default_on_abort; make it STATIC
and decorate with GC_CALLBACK; define GC_on_abort variable initialized
to GC_default_on_abort (only if SMALL_CONFIG and not PCR).
* misc.c (GC_set_abort_func, GC_get_abort_func): New public function
(only if if SMALL_CONFIG and not PCR) to alter and read GC_on_abort
value.