Complete the metadata fields for all targets. Cargo will depend on this
for checking whether a given target supports building the standard
library.

This provides a list of locations to hunt down issues in.

These constifications were blocked on classification functions being
added. Now that those methods are available, constify them.

This brings things more in line with `f32` and `f64`.

…ingjubilee

 Add classify and related methods for f16 and f128

Also constify some functions where that was blocked on classify being available.

r? libs

…metadata, r=Nilstrieb

Fill out target-spec metadata for all targets

**What does this PR try to resolve?**

This PR completes the target-spec metadata fields for all targets. This is required for a corresponding Cargo PR which adds a check for whether a target supports building the standard library when the `-Zbuild-std=std` flag is passed ([see this issue](rust-lang/wg-cargo-std-aware#87). This functionality in Cargo is reliant on the output of `--print=target-spec-json`.

**How should we test and review this PR?**

Check that a given target-spec metadata has been updated with:
```
$ ./x.py build library/std
$ build/host/stage1/bin/rustc --print=target-spec-json --target <target_name> -Zunstable-options
```

**Additional Information**

A few things to note:
* Where a targets 'std' or 'host tools' support is listed as '?' in the rust docs, these are left as 'None' with this PR. The corresponding changes in cargo will only reject an attempt to build std if the 'std' field is 'Some(false)'. In the case it is 'None', cargo will continue trying to build
* There's no rush for this to be merged. I understand that the format for this is not finalised yet.
* Related: rust-lang#120745

…td, r=jhpratt

std: `#![deny(unsafe_op_in_unsafe_fn)]` in platform-independent code

This applies the `unsafe_op_in_unsafe_fn` lint in all places in std that _do not have platform-specific cfg in their code_. For all such places, the lint remains allowed, because they need further work to address the relevant concerns. This list includes:

- `std::backtrace_rs` (internal-only)
- `std::sys` (internal-only)
- `std::os`

Notably this eliminates all "unwrapped" unsafe operations in `std::io` and `std::sync`, which will make them much more auditable in the future. Such has *also* been left for future work. While I made a few safety comments along the way on interfaces I have grown sufficiently familiar with, in most cases I had no context, nor particular confidence the unsafety was correct.

In the cases where I was able to determine the unsafety was correct without having prior context, it was obviously redundant. For example, an unsafe function calling another unsafe function that has the exact same contract, forwarding its caller's requirements just as it forwards its actual call.