[llvm-exegesis] Delegate the decision of cycle counter name to the target

Currently the cycle counter is taken from the subtarget schedule model, which
isn't any use if the subtarget doesn't have one. Delegate the decision to the
target benchmark runner, as it may know better what to do in that case, with
the default being the current behaviour.

Differential Revision: https://reviews.llvm.org/D48779

llvm-svn: 336099