Replace ABORTs followed by statement with ABORT_RET having dummy

Replace ABORTs followed by statement with ABORT_RET having dummy
condition (to suppress 'statement unreachable' compiler warnings
caused by abort tagged as 'no-return')

* dbg_mlc.c (GC_debug_realloc): Replace ABORT() with ABORT_RET().
* gcj_mlc.c (mark_stack_limit): Likewise.
* os_dep.c (catch_exception_raise_state,
catch_exception_raise_state_identity): Likewise.
* typd_mlc.c (GC_descr_obj_size, GC_push_complex_descriptor): Likewise.
* include/private/gc_priv.h (ABORT_RET): New macro.
* mark.c (GC_mark_from): Put ABORT_RET instead of "cannot happen"
comment.