bc: only check for ^C in bc_program_exec() main loop

bc: only check for ^C in bc_program_exec() main loop

Checking it in individual operations such as SQRT is not necessary
since they usually do not take very long to complete. We repeatedly return
to main loop.

TODO: re-add more fine-grained ^C checks - but only with examples where it is
actually needed. E.g. I suspect simplest ops like addition or compare won't need it.

function                                             old     new   delta
bc_program_index                                      66      64      -2
bc_num_s                                             258     246     -12
bc_num_cmp                                           294     275     -19
bc_num_compare                                        84      59     -25
bc_num_subArrays                                      74      47     -27
bc_num_a                                             474     445     -29
bc_program_exec                                     4530    4500     -30
bc_num_p                                             518     472     -46
bc_num_d                                             604     558     -46
bc_num_k                                            1005     957     -48
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/10 up/down: 0/-284)          Total: -284 bytes
   text	   data	    bss	    dec	    hex	filename
 988601	    485	   7296	 996382	  f341e	busybox_old
 988317	    485	   7296	 996098	  f3302	busybox_unstripped

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