docs: cross-testing on CI

[taiki-e]

When used together with setup-cross-toolchain-action, we can easily get coverage of the target different from the host.

It would be nice to document this in readme.

[cwfitzgerald]

An interesting thing I found when trying to go from x86 mac to aarch64 mac is that by default, llvm uses libz compression to compress part of the files. This is apparently not part of the llvm build on aarch64's llvm-tools, and you get errors about being unable to read the profraw files due to missing compression. Adding -C llvm-args=-enable-name-compression=false to RUSTFLAGS should fix that problem.