bc: convert two macros to functions, unwing one complex max(a,min(b,c))

bc: convert two macros to functions, unwing one complex max(a,min(b,c))

function                                             old     new   delta
BC_NUM_AREQ                                            -      45     +45
BC_NUM_MREQ                                            -      33     +33
bc_num_rem                                           104      91     -13
bc_num_divmod                                        168     155     -13
bc_num_d                                             584     569     -15
bc_num_mul                                            80      62     -18
bc_num_mod                                            80      62     -18
bc_num_div                                            80      62     -18
bc_num_sub                                           112      77     -35
bc_num_add                                           112      77     -35
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 0/8 up/down: 78/-165)           Total: -87 bytes
   text	   data	    bss	    dec	    hex	filename
 985526	    485	   7296	 993307	  f281b	busybox_old
 985439	    485	   7296	 993220	  f27c4	busybox_unstripped

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