Rollup merge of #129551 - RalfJung:ub-checks-fallback, r=saethlin

ub_checks intrinsics: fall back to cfg(ub_checks)

Not sure why the fallback body uses `debug_assertions`, probably a leftover from when `cfg!(ub_checks)` did not exist yet?

r? `@saethlin`

Author: matthiaskrgr