bc: convert arithmetic calc functions to "z" logic

bc: convert arithmetic calc functions to "z" logic

In order for magic macros to work, a bit of reordering was necessary.

function                                             old     new   delta
zbc_program_modexp                                     -     558    +558
zbc_program_assign                                     -     448    +448
zbc_program_pushArray                                  -     111    +111
zbc_num_s                                            239     237      -2
zbc_num_d                                            543     541      -2
zbc_num_binary                                       145     143      -2
zbc_num_a                                            443     441      -2
zbc_num_rem                                           95      91      -4
zbc_num_divmod                                       150     146      -4
zbc_num_m                                            269     257     -12
zbc_num_r                                            230     208     -22
zbc_num_printNum                                     447     417     -30
bc_program_exec                                     3960    3928     -32
zbc_num_p                                            434     399     -35
zbc_program_num                                      880     840     -40
zbc_num_k                                            913     852     -61
zbc_program_printStream                              141       -    -141
bc_program_assign                                    455       -    -455
bc_program_modexp                                    668       -    -668
------------------------------------------------------------------------------
(add/remove: 3/3 grow/shrink: 0/13 up/down: 1117/-1512)      Total: -395 bytes
   text	   data	    bss	    dec	    hex	filename
 982632	    485	   7296	 990413	  f1ccd	busybox_old
 982237	    485	   7296	 990018	  f1b42	busybox_unstripped

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