Add support for FUTEX_{WAIT,WAKE}_BITSET
FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET are extensions of FUTEX_WAIT and FUTEX_WAKE that allow tagging each waiting thread with up to 32 'labels', and then only wake up threads that match certain labels. The non-bitset operations behave like their bitset was fully set (u32::MAX), meaning that they'll wait for anything, and wake up anything.
The only other difference is that FUTEX_WAIT_BITSET uses an absolute timeout instead of an relative timeout like FUTEX_WAIT.
Often, FUTEX_WAIT_BITSET is used not for its bitset functionality, but only for its absolute timeout functionality. It is then used with a bitset of u32::MAX.
~~This adds support for only that use case to Miri, as that's all `std` currently needs. Any other bitset is still unsupported.~~
Update: This adds full support for both these syscalls.
make strict-provenance imply check-number-validity
I feel like Miri not catching [this example](https://github.com/rust-lang/unsafe-code-guidelines/issues/286#issuecomment-1085144431) with strict provenance checking enabled is surprising.
OTOH, Miri suddenly complaining about uninit data in integers with `-Zmiri-strict-provenance` also might be surprising. Which one is more surprising? I don't know. We *could* go out of our way and have a mode where uninit integers are okay but provenance is not, but I am not sure if that is truly worth it. It'd be quite annoying to implement.
add -Zmiri-strict-provenance
This implements [strict provenance](https://github.com/rust-lang/rust/issues/95228) in Miri. The only change is that casting an integer to a pointer does not even attempt to produce a good provenance for the given address; instead, it always uses the invalid provenance. This stricter than even `-Zmiri-tag-raw-pointers` in that it also rejects the following example (which does not even involve Stacked Borrows):
```rust
fn main() {
let x = 22;
let ptr = &x as *const _ as *const u8;
let roundtrip = ptr as usize as *const u8;
let _ = unsafe { roundtrip.offset(1) };
}
```
The new flag also implies `-Zmiri-tag-raw-pointers` since the only reason one would *not* want to tag raw pointers is to support ptr-int-ptr roundtrips.
Note that the flag does *not* check against ptr-to-int *transmutes*; that still requires `-Zmiri-check-number-validity`. You can also check for strict provenance *without* Stacked Borrows by adding `-Zmiri-disable-stacked-borrows`.
The new "Miri hard mode" flags for maximal checking are `-Zmiri-strict-provenance -Zmiri-check-number-validity`. (Add `-Zmiri-symbolic-alignment-check` if you feel extra spicy today.)
ensure that -Zmiri-check-number-validity detects integers with provenance
This actually currently *fails* for the non-array case; I will have to fix this on the rustc side.
another test for too big type
The existing test covers "slice is bigger than largest supported object" but we had no test covering "total size is bigger than largest supported object", which happens when the unsized tail itself is okay in terms of size, but together with the sized prefix it becomes too big.
Cc https://github.com/rust-lang/rust/issues/95334
