Add maxperf build profile (#3608)

## Proposed Changes

Add a new Cargo compilation profile called `maxperf` which enables more aggressive compiler optimisations at the expense of compilation time.

Some rough initial benchmarks show that this can provide up to a 25% reduction to run time for CPU bound tasks like block processing: https://docs.google.com/spreadsheets/d/15jHuZe7lLHhZq9Nw8kc6EL0Qh_N_YAYqkW2NQ_Afmtk/edit

The numbers in that spreadsheet compare the `consensus-context` branch from #3604 to the same branch compiled with the `maxperf` profile using:

```
PROFILE=maxperf make install-lcli
```

## Additional Info

The downsides of the maxperf profile are:

- It increases compile times substantially, which will particularly impact low-spec hardware. Compiling `lcli` is about 3x slower. Compiling Lighthouse is about 5x slower on my 5950X: 17m 38s rather than 3m 28s.

As a result I think we should not enable this everywhere by default.

- **Option 1**: enable by default for our released binaries. This gives the majority of users the fastest version of `lighthouse` possible, at the expense of slowing down our release CI. Source builds will continue to use the default `release` profile unless users opt-in to `maxperf`.
- **Option 2**: enable by default for source builds. This gives users building from source an edge, but makes them pay for it with compilation time. 

I think I would prefer Option 1. I'll try doing some benchmarking to see how long a maxperf build of Lighthouse would take on GitHub actions.

Credit to Nicholas Nethercote for documenting these options in the Rust Performance Book: https://nnethercote.github.io/perf-book/build-configuration.html.

book/src/cross-compiling.md

@@ -44,3 +44,9 @@ in `lighthouse/target/aarch64-unknown-linux-gnu/release`.
 When using the makefile the set of features used for building can be controlled with
 the environment variable `CROSS_FEATURES`. See [Feature
  Flags](./installation-source.md#feature-flags) for available features.
+
+## Compilation Profiles
+
+When using the makefile the build profile can be controlled with the environment variable
+`CROSS_PROFILE`. See [Compilation Profiles](./installation-source.md#compilation-profiles) for
+available profiles.