Cross-compiling aya-based programs

The instructions below show how to cross compile aya eBPF programs on Macs. Cross compiling on other systems is possible too and we're going to add instructions to do that soon (PRs welcome!).

Cross-compiling aya-based programs on Mac

Cross compilation should work on both Intel and Apple Silicon Macs.

Install rustup following the instructions on https://rustup.rs/

Install the stable and nightly rust toolchains:

rustup install stable
rustup toolchain install nightly --component rust-src

Install the rustup target for your Linux target platform:

ARCH=x86_64
rustup target add ${ARCH}-unknown-linux-musl

Install LLVM with brew:
```
brew install llvm
```
Install the musl cross compiler:
to cross-compile for only x86_64 targets (the default in musl-cross):
```
brew install FiloSottile/musl-cross/musl-cross
```
to cross-compile for only aarch64 targets:
```
brew install FiloSottile/musl-cross/musl-cross --without-x86_64 --with-aarch64
```
to cross-compile for both x86_64 and aarch64 targets:
```
brew install FiloSottile/musl-cross/musl-cross --with-aarch64
```
See homebrew-musl-cross for additional platform-specific options.

Install bpf-linker:

LLVM_SYS_160_PREFIX=$(brew --prefix llvm) cargo install bpf-linker --no-default-features

Build BPF object files:
```
cargo xtask build-ebpf --release
```
Build the userspace code:
```
RUSTFLAGS="-Clinker=${ARCH}-linux-musl-ld" cargo build --release --target=${ARCH}-unknown-linux-musl
```
The cross-compiled program
target/${ARCH}-unknown-linux-musl/release/<program_name>
can be copied to a Linux server or VM and run there.