bc: select parse_init() and parse_expr() using IS_BC, not function pointers

bc: select parse_init() and parse_expr() using IS_BC, not function pointers

function                                             old     new   delta
common_parse_expr                                      -      62     +62
common_parse_init                                      -      29     +29
dc_parse_parse                                        53      52      -1
dc_parse_expr                                        723     719      -4
bc_program_execStr                                   606     594     -12
dc_parse_init                                         33      18     -15
bc_parse_init                                         33      18     -15
bc_parse_expression                                   39       -     -39
bc_vm_run                                           1923    1872     -51
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/6 up/down: 91/-137)           Total: -46 bytes
   text	   data	    bss	    dec	    hex	filename
 988774	    485	   7296	 996555	  f34cb	busybox_old
 988728	    485	   7296	 996509	  f349d	busybox_unstripped

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