bc: compress two constant arguments of bc_lex_assign() to one

bc: compress two constant arguments of bc_lex_assign() to one

function                                             old     new   delta
zbc_program_num                                      836     835      -1
bc_lex_assign                                         34      31      -3
zbc_lex_next                                        1930    1880     -50
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-54)             Total: -54 bytes
   text	   data	    bss	    dec	    hex	filename
 980208	    485	   7296	 987989	  f1355	busybox_old
 980154	    485	   7296	 987935	  f131f	busybox_unstripped

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