Use prebuilt gnullvm target

[mati865]

With rust-lang/rust#121712 merged we can drop -Z build-std workaround.

[mati865]

I think it will work when new nightly is published and CI is retried.

[mati865]

Blocked on rust-lang/rust#124229

[jeremyd2019]

I was hoping it would reduce the need for special-case steps, like for instance the Test and Test gnullvm steps are now the same. Note that llvm-mingw does also ship a mac toolchain, so theoretically could install that and get rid of more special if checks for ubuntu...

[mati865]

I've experimented with replacing duplicates with more complicated if conditions but it's not that easy. I think the best way to handle it is to leave duplicates until 13 June when current nightly will become stable.

Note that llvm-mingw does also ship a mac toolchain, so theoretically could install that and get rid of more special if checks for ubuntu...

I was wondering whether it would be beneficial to add it but I don't see much value in mac builds (windows -> windows-gnullvm could me more useful tough).

[mati865]

Marked as ready to review as the CI should pass with today's nightly.