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/SUMMARY.md

@@ -7,6 +7,7 @@
 * [Installation](./installation.md)
     * [Docker](./docker.md)
     * [Raspberry Pi 4](./pi.md)
+    * [Cross-Compiling](./cross-compiling.md)
 * [Key Management](./key-management.md)
     * [Create a wallet](./wallet-create.md)
     * [Create a validator](./validator-create.md)

book/src/become-a-validator-source.md

@@ -3,7 +3,9 @@
 ## 0. Install Rust
 If you don't have Rust installed already, visit [rustup.rs](https://rustup.rs/) to install it.
 
-> Note:  if you're not familiar with Rust or you'd like more detailed instructions, see our  [installation guide](./installation.md).
+> Notes:
+>   - If you're not familiar with Rust or you'd like more detailed instructions, see our  [installation guide](./installation.md).
+>   - Windows is presently only supported via [WSL](https://docs.microsoft.com/en-us/windows/wsl/about).
 
 
 ## 1. Download and install Lighthouse

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`.

book/src/pi.md

@@ -5,6 +5,11 @@ Tested on:
  - Raspberry Pi 4 Model B (4GB)
  - `Ubuntu 20.04 LTS (GNU/Linux 5.4.0-1011-raspi aarch64)`
 
+
+*Note: [Lighthouse supports cross-compiling](./cross-compiling.md) to target a
+Raspberry Pi (`aarch64`). Compiling on a faster machine (i.e., `x86_64`
+desktop) may be convenient.*
+
 ### 1. Install Ubuntu
 
 Follow the [Ubuntu Raspberry Pi installation instructions](https://ubuntu.com/download/raspberry-pi).