Add support for UEFI targets and OS

[lorenz]

Requires bazelbuild/platforms#76

[UebelAndre]

Seems fine for whenever the upstream PR merges 😄

Thank you!

[amari]

I've opened a new upstream PR (bazelbuild/platforms#97) since the original one seems to be abandoned.

[lorenz]

That PR is not really abandoned, it has approval and is waiting for someone on the Bazel team to merge it (I'm the person who did both PRs).

[lorenz]

I'll rebase this either this week or early next one, then we can merge it now that the platform constraint has been merged upstream.

[mikea]

I can't figure out how to use rules_rust with this change in: it completely breaks bazel 8.0.1 with

 no such target '@@platforms//os:uefi': target 'uefi' not declared in package 'os'

message, despite me adding fresh "platforms" http_archive repository according to platforms release notes. It seems that bazel insits on using built-in platforms, which makes new versions of rules_rust not usable until bazel incorporates uefi platform changes into the release.

I was able to overcome this by adding "platforms" to MODULE.bazel instead. Sorry for the noise.