Add Rust macOS instructions to README

[nickgerace]

Since version 1.4.1 contains alpha support for macOS, it may be a good idea to show Rust users how to try it.

[rui314]

Please add a signed-off-by line to your commit message as instructed by https://github.com/rui314/mold/blob/main/CONTRIBUTING.md.

As to this change, I actually wonder if you can generalize

[target.x86_64-unknown-linux-gnu]

and

[target.aarch64-apple-darwin]

lines. Isn't there any way to just pass a linker flag in all targets?

[nickgerace]

Oops! I'm sorry. I usually sign my commits. I made this PR from a different machine than usual, my mistake.

EDIT: nevermind, the commit already looks signed on my end. Does that suffice?

As for the change: I believe you can, but those targets can include Windows and the path to ld64.mold or mold might be different depending on the distribution or OS.

What we might be able to do is something like the following...

[target.your-linux-or-darwin-target]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold"]

And then add a comment reminding the user that ld64.mold should be specified for building darwin/macOS binaries.

Thoughts?

[rui314]

OK, I think if we can't generalize it, we should write the actual targets. I want to make it something that you can just copy-n-paste, so replacing it with target.your-linux-or-darwin-target doesn't seem like a good idea.

[nickgerace]

Thank you! I'll keep an eye out to see if this can be improved in the future too.