bc: factor out common code

bc: factor out common code

function                                             old     new   delta
zbc_parse_stmt_allow_NLINE_before                      -      59     +59
bc_parse_pushJUMP_ZERO                                 -      27     +27
bc_parse_pushJUMP                                      -      27     +27
rewrite_label_to_current                               -      19     +19
zbc_vm_process                                       594     599      +5
zbc_lex_next_and_skip_NLINE                           22       -     -22
zbc_parse_stmt_fail_if_bare_NLINE                     28       -     -28
zbc_parse_stmt_possibly_auto                        1909    1678    -231
------------------------------------------------------------------------------
(add/remove: 4/2 grow/shrink: 1/1 up/down: 137/-281)         Total: -144 bytes
   text	   data	    bss	    dec	    hex	filename
 981879	    485	   7296	 989660	  f19dc	busybox_old
 981755	    485	   7296	 989536	  f1960	busybox_unstripped

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