Simplify redefinition of 'abort' (Bug#12316).

Simplify redefinition of 'abort' (Bug#12316).

Do not try to redefine the 'abort' function.  Instead, redo
the code so that it calls 'emacs_abort' rather than 'abort'.
This removes the need for the NO_ABORT configure-time macro
and makes it easier to change the abort code to do a backtrace.
* configure.ac (NO_ABRT): Remove.
* admin/CPP-DEFINES (NO_ABORT): Remove.
* nt/inc/ms-w32.h (w32_abort) [HAVE_NTGUI]: Remove.
* src/.gdbinit: Just stop at emacs_abort, not at w32_abort or abort.
* src/emacs.c (abort) [!DOS_NT && !NO_ABORT]:
Remove; sysdep.c's emacs_abort now takes its place.
* src/lisp.h (emacs_abort): New decl.  All calls from Emacs code to
'abort' changed to use 'emacs_abort'.
* src/msdos.c (dos_abort) [defined abort]: Remove; not used.
(abort) [!defined abort]: Rename to ...
(emacs_abort): ... new name.
* src/sysdep.c (emacs_abort) [!HAVE_NTGUI]: New function, taking
the place of the old 'abort' in emacs.c.
* src/w32.c, src/w32fns.c (abort): Do not #undef.
* src/w32.c (emacs_abort): Rename from w32_abort.