*: stop using atexit in non-debug build: saves ~260 in bss with musl

*: stop using atexit in non-debug build: saves ~260 in bss with musl

"builtin" below is atexit's 32-element global array of functions to call.

function                                             old     new   delta
top_main                                             879     889     +10
launch_helper                                        185     193      +8
powertop_main                                       1555    1559      +4
sed_main                                             651     650      -1
slot                                                   4       -      -4
call                                                   4       -      -4
atexit                                                23       -     -23
kill_helper                                           31       -     -31
__funcs_on_exit                                      120       -    -120
__cxa_atexit                                         168       -    -168
builtin                                              260       -    -260
------------------------------------------------------------------------------
(add/remove: 0/8 grow/shrink: 3/1 up/down: 22/-611)          Total: -589 bytes
   text	   data	    bss	    dec	    hex	filename
 912364	    563	   6132	 919059	  e0613	busybox_old
 912035	    563	   5844	 918442	  e03aa	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>