Set -O2 via NoFibHcOpts instead of individual Makefiles.
We set -O2 in NoFibHcOpts which is then applied to all benchmarks run.
Adding -O2 in individual benchmarks is therefore redundant.
It also leads to issues when testing performance flags via
Individual Makefiles attach -O2 last. This means all
flags set by -O2 could not be disabled with -fno-<flag> as they
were reenabled by the later -O2 switch.
Test Plan: Using
Reviewers: bgamari, jmct, O26 nofib
Differential Revision: https://phabricator.haskell.org/D4829