[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