Cross-compile to vendored x86_84, aarch64 (Raspberry Pi 4) (#1497)

## Issue Addressed

NA

## Proposed Changes

Adds support for using the [`cross`](https://github.com/rust-embedded/cross) project to produce cross-compiled binaries using Docker images.

Provides quite clean and simple cross-compiles cause all the complexity is hidden in Dockerfiles. It does require you to be in the `docker` group though.

## Details

- Adds shortcut commands to `Makefile`
- Ensures `reqwest` and `discv5` use vendored openssl libs (i.e., static not shared).
- Switches to a [commit](284f705964) of blst that has a renamed C function to avoid a collision with openssl (upstream issue: https://github.com/supranational/blst/issues/21).
- Updates `ring` to the latest satisfiable version, since an earlier version was causing issues with `cross`.
- Off-topic, but adds extra message about Windows support as suggested by Discord user.

## Additional Info

- ~~Blocked on #1495~~
- There are no tests in CI for this yet for a few reasons:
  - I'm hesitant to add more long-running tasks.
  - Short-term bitrot should be avoided since we'll use it each release.
  - In the long term I think it would be good to automate binary creation on a release.
- I observed the binaries increase in size from 50mb to 52mb after these changes.

book/src/cross-compiling.md

@@ -0,0 +1,41 @@
+# Cross-compiling
+
+Lighthouse supports cross-compiling, allowing users to run a binary on one
+platform (e.g., `aarch64`) that was compiled on another platform (e.g.,
+`x86_64`).
+
+
+## Instructions
+
+Cross-compiling requires [`Docker`](https://docs.docker.com/engine/install/),
+[`rustembedded/cross`](https://github.com/rust-embedded/cross) and for the
+current user to be in the `docker` group.
+
+The binaries will be created in the `target/` directory of the Lighthouse
+project.
+
+### Targets
+
+The `Makefile` in the project contains four targets for cross-compiling:
+
+- `build-x86_64`: builds an optimized version for x86_64 processors (suitable
+	for most users).
+- `build-x86_64-portable`: builds a version x86_64 processors which avoids
+	using some modern CPU instructions that might cause an "illegal
+	instruction" error on older CPUs.
+- `build-aarch64`: builds an optimized version for 64bit ARM processors
+	(suitable for Raspberry Pi 4).
+- `build-aarch64-portable`: builds a version 64 bit ARM processors which avoids
+	using some modern CPU instructions that might cause an "illegal
+	instruction" error on older CPUs.
+
+
+### Example
+
+```bash
+cd lighthouse
+make build-aarch64
+```
+
+The `lighthouse` binary will be compiled inside a Docker container and placed
+in `lighthouse/target/aarch64-unknown-linux-gnu/release`.