std: unconditionally use preadv/pwritev on AArch64 macOS

[joboet]

The stars happened to line up perfectly: macOS 11.0 was the first version to support ARM, and also the first version with preadv/pwritev. Thus we don't need to use weak linkage if the target is aarch64-apple-darwin.

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[Mark-Simulacrum]

How did you check which versions of macOS support preadv/pwritev? Maybe we can document that into https://doc.rust-lang.org/nightly/rustc/platform-support/apple-darwin.html or some other location (seems like more generally useful information).

Also -- do we know if the non-macOS platforms support these (watchos/tvos/etc)?