bc: get rid of G.prog.ob, G.prog.strmb

bc: get rid of G.prog.ob, G.prog.strmb

function                                             old     new   delta
zbc_num_printNum                                     489     540     +51
zbc_program_asciify                                  426     473     +47
zbc_program_print                                    686     684      -2
zbc_program_exec                                    4008    3995     -13
zbc_program_assign                                   474     440     -34
bc_vm_init                                           739     663     -76
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/4 up/down: 98/-125)           Total: -27 bytes
   text	   data	    bss	    dec	    hex	filename
 981404	    485	   7296	 989185	  f1801	busybox_old
 981377	    485	   7296	 989158	  f17e6	busybox_unstripped

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