[llvm-exegesis] benchmarkMain(): less cryptic error if built w/o libpfm

Wanted to check if inablility to measure latency of CMOV32rm
is a regression from D60041 / D60138, but unable to do that
because the llvm-exegesis-{8,9} from debian sid fails
with that cryptic, unhelpful error.

I suspect this will be a better error.

llvm-svn: 357900

llvm/tools/llvm-exegesis/llvm-exegesis.cpp

@@ -359,6 +359,11 @@ readSnippets(const LLVMState &State, llvm::StringRef Filename) {
 }
 
 void benchmarkMain() {
+#ifndef HAVE_LIBPFM
+  llvm::report_fatal_error(
+      "benchmarking unavaliable, LLVM was built without libpfm.");
+#endif
+
   if (exegesis::pfm::pfmInitialize())
     llvm::report_fatal_error("cannot initialize libpfm");