Rollup merge of #137306 - tgross35:remove-i128-u128-improper-ctypes, r=traviscross,workingjubilee

Remove `i128` and `u128` from `improper_ctypes_definitions`

Rust's 128-bit integers have historically been incompatible with C [1]. However, there have been a number of changes in Rust and LLVM that mean this is no longer the case:

* Incorrect alignment of `i128` on x86 [1]: adjusting Rust's alignment proposed at rust-lang/compiler-team#683, implemented at rust-lang/rust#116672.
* LLVM version of the above: resolved in LLVM, including ABI fix. Present in LLVM18 (our minimum supported version).
* Incorrect alignment of `i128` on 64-bit PowerPC, SPARC, and MIPS [2]: Rust's data layouts adjusted at rust-lang/rust#132422, rust-lang/rust#132741, rust-lang/rust#134115.
* LLVM version of the above: done in LLVM 20 llvm/llvm-project#102783.
* Incorrect return convention of `i128` on Windows: adjusted to match GCC and Clang at rust-lang/rust#134290.

At rust-lang/lang-team#255 (comment), the lang team considered it acceptable to remove `i128` from `improper_ctypes_definitions` if the LLVM version is known to be compatible. Time has elapsed since then and we have dropped support for LLVM versions that do not have the x86 fixes, meaning a per-llvm-version lint should no longer be necessary. The PowerPC, SPARC, and MIPS changes only came in LLVM 20 but since Rust's datalayouts have also been updated to match, we will be using the correct alignment regardless of LLVM version.

`repr(i128)` was added to this lint in rust-lang/rust#138282, but is also removed here.

Part of the decision is that `i128` should match `__int128` in C on platforms that provide it, which documentation is updated to indicate. We will not guarantee that `i128` matches `_BitInt(128)` since that can be different from `__int128`. Some platforms (usually 32-bit) do not provide `__int128`; if any ABIs are extended in the future to define it, we will need to make sure that our ABI matches.

Closes: rust-lang/rust#134288

[1]: rust-lang/rust#54341
[2]: rust-lang/rust#128950

Author: matthiaskrgr