Add documentation for cross-compiling to AWS arm

When cross-compiling for AWS graviton Arm processors, you have to specify the appropriate target CPU or you may end up getting a hard-to-diagnose Illegal Instruction (SIGILL, exit code 132) error.

As this is a common use case, a note in the readme would be helpful.

Example:
```bash
RUSTFLAGS='-C target-cpu=neoverse-n1' cargo zigbuild --release --target aarch64-unknown-linux-gnu.2.34
```