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.lock

@@ -1,5 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "account_manager"
 version = "0.3.5"
@@ -3756,6 +3758,7 @@ dependencies = [
  "slog-async",
  "slog-term",
  "sloggers",
+ "target_check",
  "task_executor",
  "tempfile",
  "tokio 1.5.0",
@@ -6331,6 +6334,13 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "55937e1799185b12863d447f42597ed69d9928686b8d88a1df17376a097d8369"
 
+[[package]]
+name = "target_check"
+version = "0.1.0"
+dependencies = [
+ "static_assertions",
+]
+
 [[package]]
 name = "target_info"
 version = "0.1.0"