Support `arm64e-apple-darwin`

[madsmtm]

See the platform support docs and rust-lang/rust#73628.

We currently error with:

error occurred: Unknown architecture for macOS target

This is affecting bootstrap, i.e. you cannot do ./x build --target=arm64e-apple-darwin.

(Note that the target may be renamed in the future, but we should support it with its current name).

CC target maintainer @arttet.