bc: do not use "(cond ? f1 : f2)(params)" idiom, it messes up static function optimiz...

bc: do not use "(cond ? f1 : f2)(params)" idiom, it messes up static function optimizations

With direct calls, GCC no longer thinks that we take addresses of the functions,
and can use "more optimal" internal ABI for _all_ calls to these functions,
not only at this callsite. On i486, regparm is used, and:

function                                             old     new   delta
zbc_num_inv                                           56      57      +1
zbc_num_k                                            852     851      -1
zbc_program_modexp                                   558     556      -2
zbc_num_d                                            541     539      -2
bc_num_ulong2num                                      59      57      -2
zbc_program_num                                      840     836      -4
bc_num_zero                                           11       7      -4
bc_num_one                                            28      24      -4
bc_program_exec                                     3928    3918     -10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/8 up/down: 1/-29)             Total: -28 bytes
   text	   data	    bss	    dec	    hex	filename
 982237	    485	   7296	 990018	  f1b42	busybox_old
 982209	    485	   7296	 989990	  f1b26	busybox_unstripped

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