Split GC_abort with GC_on_abort and abort() invoked from ABORT

Split GC_abort with GC_on_abort and abort() invoked from ABORT
(to let ABORT inherit 'noreturn' attribute of abort)

* misc.c (GC_abort): Replace with GC_on_abort (return from GC_on_abort
instead of abort); update the comment.
* include/private/gc_priv.h (GC_abort): Likewise.
* include/private/gc_priv.h (GC_on_abort): Define as empty macro in
case of SMALL_CONFIG.
* include/private/gc_priv.h (ABORT): Always call GC_on_abort (unless
PCR) and abort (or _exit, or ExitProcess or DebugBreak, depending on
the target).