bc: add and use bc_num_init_DEF_SIZE()

bc: add and use bc_num_init_DEF_SIZE()

function                                             old     new   delta
bc_num_init_DEF_SIZE                                   -      10     +10
bc_num_binary                                        150     148      -2
bc_program_pushVar                                   208     203      -5
bc_program_modexp                                    741     736      -5
bc_program_copyToVar                                 316     311      -5
bc_program_call                                      352     347      -5
bc_array_expand                                       88      83      -5
bc_program_num                                      1125    1115     -10
bc_vm_init                                           821     786     -35
bc_program_exec                                     4152    4098     -54
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/9 up/down: 10/-126)          Total: -116 bytes
   text	   data	    bss	    dec	    hex	filename
 985988	    477	   7296	 993761	  f29e1	busybox_old
 985872	    477	   7296	 993645	  f296d	busybox_unstripped

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