[perf2bolt] Enforce strict mode for perf2bolt

Summary:
In strict relocation mode, we get better function coverage. However, if
the profile used for optimization was converted using non-strict mode,
then it wouldn't match functions exclusive to strict mode. Hence,
we have to enforce strict relocation mode for profile conversion, so it
can be used for either mode.

I'm also adding parallel profile pre-processing unless `--no-threads` is
specified. This masks the runtime overhead of function disassembly on
multi-core machines.

(cherry picked from FBD16587855)