Prevent unsoundness in environments with broken `madvise(MADV_DONTNEED)` (#11722)

* Prevend unsoundness in environments with broken `madvise(MADV_DONTNEED)`

* Add the `std` feature to `rustix` dependency

Apparently not having this breaks compilation on non-nightly toolchains.

* Autodetect the page size when checking whether `madvise` works

* Only make sure that the madvice check doesn't return `Err`