Constant pointers to Byfl data

Prepared Byfl for run-time performance improvement by declaring all
global pointers as type "uint64_t * const" instead of just "uint64_t
*".  (This currently applies to bf_mem_insts_count, bf_inst_mix_histo,
bf_terminator_count, and bf_mem_intrin_count.)  Doing so enables the
optimizer to merge increments to array values by asserting that the
pointer cannot change between increments.