[MC][TableGen] Add optional libpfm counter names for ProcResUnits.

Summary:
Subtargets can define the libpfm counter names that can be used to
measure cycles and uops issued on ProcResUnits.
This allows making llvm-exegesis available on more targets.
Fixes PR36984.

Reviewers: gchatelet, RKSimon, andreadb, craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45360

llvm-svn: 329675