Fix build error on latest nightly

[toku-sa-n]

See: rust-lang/rust#77134

[josephlr]

Hopefully we will reach a point where const_fn is split up enough that we won't have to depend on the blanket feature.

[josephlr]

This passes locally for me, but the issue seems to be that the test framework builds with the old version of this crate, so the tests can never pass.

[josephlr]

@phil-opp I don't have permissions to override the CI, otherwise I would merge this.

[phil-opp]

Thank you!

The CI fails because the test framework depends on the bootloader crate, which itself has a dependency on an older x86_64 version. I don't think that we can resolve this without publishing new x86_64 and bootloader versions unfortunately, so let's merge and publish this even with failing CI. For the future we should try to remove this circular dependency if possible.

[phil-opp]

Published as v0.12.1. I'll do the bootloader update next, to hopefully fix the CI again.

[phil-opp]

I published versions of the bootloader (in rust-osdev/bootloader#128) and uart_16550 crates and manually restarted the CI. Looks like everything is green again: https://github.com/rust-osdev/x86_64/runs/1159219587