[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
diff --git a/llvm/lib/Target/X86/X86.td b/llvm/lib/Target/X86/X86.td
index a701788..c88f3a5 100644
--- a/llvm/lib/Target/X86/X86.td
+++ b/llvm/lib/Target/X86/X86.td
@@ -1132,3 +1132,9 @@
   let AssemblyWriters = [ATTAsmWriter, IntelAsmWriter];
   let AllowRegisterRenaming = 1;
 }
+
+//===----------------------------------------------------------------------===//
+// Pfm Counters
+//===----------------------------------------------------------------------===//
+
+include "X86PfmCounters.td"