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
EXTRA_HC_OPTS.
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