Linux: Nuke `Stat` bits in favour of statx

Maintaining the POSIX `stat` bits for Zig is a pain. The order and
bit-length of members differ between all architectures, and int types
can be signed or unsigned. The libcs deal with this by introducing the
own version of `struct stat` and copying the kernel structure members to
it. In the case of glibc, they did it twice thanks to the largefile
transition!

In practice, the project needs to maintain three versions of `struct
stat`:
- What the kernel defines.
- What musl wants for `struct stat`.
- What glibc wants for `struct stat64`. Make sure to use `fstatat64`!

This isn't as simple as running `zig translate-c`. In #21440 I had to:
- Compile toolchains for each arch+glibc/musl combo.
- Create a test `fstat` program with/without `FILE_OFFSET_BITS=64`.
- Dump the value for `struct stat`.
- Stare at `std.os.linux`/`std.c` and cry.
- Add some missing padding.

The fact that so many target checks in the `linux` and `posix` tests
exist is most likely due to writing to padding bits and failing later.

The solution to this madness is `statx(2)`:
- It takes a single structure that is the same for all arches AND libcs.
- It uses a custom timestamp format, but it is 64-bit ready.
- It gives the same info as `fstatat(2)` and more!
- Unlike `fstatat(2)`, you can request a subset of the info required
  based on passing a mask.

It's so good that modern Linux arches (e.g. riscv) don't even implement
`stat`, with the libcs using a generic `struct stat` and copying from
`struct statx`.

Therefore, this commit rips out all the `stat` bits from `std.os.linux`
and `std.c`. `std.posix.Stat` is now `void`, and calling
`std.posix.*stat` is an compile-time error. A wrapper around `statx` has
been added to `std.os.linux`, and callers have been upgraded to use it.
Tests have also been updated to use `statx` where possible.

While I was here, I converted the mask and file attributes to be packed
struct bitfields. A nice side effect is checking that you actually
recieved the members you asked for via `Statx.mask`, which I have used
by adding `assert`s at specific callsites.

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