Improve compilation error on 32-bit (#2424)

## Issue Addressed

Closes #1661

## Proposed Changes

Add a dummy package called `target_check` which gets compiled early in the build and fails if the target is 32-bit

## Additional Info

You can test the efficacy of this check with:

```
cross build --release --manifest-path lighthouse/Cargo.toml --target i686-unknown-linux-gnu
```

In which case this compilation error is shown:

```
error: Lighthouse requires a 64-bit CPU and operating system
  --> common/target_check/src/lib.rs:8:1
   |
8  | / assert_cfg!(
9  | |     target_pointer_width = "64",
10 | |     "Lighthouse requires a 64-bit CPU and operating system",
11 | | );
   | |__^
```

Cargo.toml

@@ -37,6 +37,7 @@ members = [
     "common/sensitive_url",
     "common/slot_clock",
     "common/task_executor",
+    "common/target_check",
     "common/test_random_derive",
     "common/validator_dir",
     "common/warp_utils",

common/target_check/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "target_check"
+version = "0.1.0"
+authors = ["Michael Sproul <[email protected]>"]
+edition = "2018"
+
+[dependencies]
+static_assertions = "1.1.0"

common/target_check/src/lib.rs

@@ -0,0 +1,11 @@
+//! This crate checks properties of the target architecture to ensure that it's compatible with
+//! Lighthouse.
+use static_assertions::assert_cfg;
+
+// In many places we assume `usize` and `u64` have the same in-memory representation.
+// We also use memory-mapped files extensively which are only really viable with 64-bit addressing.
+// It's unlikely we will want to support 128-bit architectures any time soon.
+assert_cfg!(
+    target_pointer_width = "64",
+    "Lighthouse requires a 64-bit CPU and operating system",
+);

lighthouse/Cargo.toml

@@ -47,6 +47,7 @@ lazy_static = "1.4.0"
 serde_json = "1.0.59"
 task_executor = { path = "../common/task_executor" }
 malloc_utils = { path = "../common/malloc_utils" }
+target_check = { path = "../common/target_check" }
 
 [dev-dependencies]
 tempfile = "3.1.0"
@@ -57,3 +58,8 @@ eth2_libp2p = { path = "../beacon_node/eth2_libp2p" }
 [[test]]
 name = "lighthouse_tests"
 path = "tests/main.rs"
+
+# Prevent cargo-udeps from flagging the dummy package `target_check`, which exists only
+# to assert properties of the compilation target.
+[package.metadata.cargo-udeps.ignore]
+normal = ["target_check"]