rust: fix host build on aarch64 darwin

rust: fix host build on aarch64 darwin

rust/host failed to compile on macOS running on Apple Silicon M1 Pro
because the host target triple is autogenerated to be
'arm64-unknown-linux-'. Rust doesn't have such a target triple, thus the
build failes because there are no pre-built artifacts for bootstrapping.

Fix this by setting RUSTC_HOST_ARCH to 'aarch64-apple-darwin' in case
our host is HOST_ARCH=arm64 and HOST_OS=darwin.

Signed-off-by: Jonas Jelonek <redacted>