Refactor target triple handling

[messense]

Currently maturin build --target xxx --universal2 would results in two --target xxx passed to cargo rustc which errored with:

error: specifying multiple `--target` flags requires `-Zmultitarget`
💥 maturin failed
  Caused by: Failed to build a native library through cargo
  Caused by: Failed to build a aarch64 library through cargo
  Caused by: Cargo build finished with "exit code: 101": `cargo rustc --message-format json --manifest-path Cargo.toml --target x86_64-apple-darwin --release --target aarch64-apple-darwin --bins --`

This patch changes how target is passed to compile_target function by remove it from cargo_extra_args and pass it directly into compile_target function.

[konstin]

I hope that this is really the same target as cargo uses and we don't start cross-compiling where we wouldn't need to 🤞

[konstin]

Thanks!