bc: support void functions (GNU compat)

bc: support void functions (GNU compat)

function                                             old     new   delta
xc_program_print                                       -     689    +689
zxc_vm_process                                       814     869     +55
zxc_program_exec                                    4098    4116     +18
zxc_program_assign                                   385     392      +7
bc_result_free                                        43      46      +3
zxc_program_binOpPrep                                243     245      +2
zdc_program_execStr                                  518     520      +2
zxc_program_print                                    683       -    -683
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 6/0 up/down: 776/-683)           Total: 93 bytes

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