[llvm-exegesis][NFC] Add a way to declare the default counter binding for...

[llvm-exegesis][NFC] Add a way to declare the default counter binding for unbound CPUs for a target.

Summary:
This simplifies the code and moves everything to tablegen for consistency. This
also prepares the ground for adding issue counters.

Reviewers: gchatelet, john.brawn, jsji

Subscribers: nemanjai, mgorny, javed.absar, kbarton, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54297

llvm-svn: 346489