bc: convert to "G trick" - this returns bc to zero bss increase

bc: convert to "G trick" - this returns bc to zero bss increase

function                                             old     new   delta
bc_num_p                                             518     540     +22
bc_num_k                                            1010    1031     +21
bc_vm_process                                        312     327     +15
bc_program_exec                                     4401    4413     +12
bc_vm_posixError                                     194     205     +11
bc_num_a                                             480     491     +11
bc_program_reset                                     182     192     +10
bc_vm_sig                                             88      97      +9
bc_vm_error                                          148     156      +8
bc_parse_expr                                       2215    2222      +7
bc_num_compare                                        84      90      +6
dc_parse_parse                                        53      58      +5
dc_lex_token                                         670     675      +5
bc_read_line                                         359     364      +5
bc_parse_parse                                       471     476      +5
bc_num_cmp                                           294     299      +5
bc_num_subArrays                                      84      82      -2
bc_num_d                                             614     609      -5
dc_main                                               80      72      -8
bc_main                                               80      72      -8
bcg                                                   40       -     -40
bc_vm_run                                           2569    2382    -187
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 16/5 up/down: 157/-250)         Total: -93 bytes
   text	   data	    bss	    dec	    hex	filename
 989425	    485	   7336	 997246	  f377e	busybox_old
 989372	    485	   7296	 997153	  f3721	busybox_unstripped

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