Auto merge of #90791 - drmorr0:drmorr-memcmp-cint-cfg, r=petrochenkov

make memcmp return a value of c_int_width instead of i32

This is an attempt to fix #32610 and #78022, namely, that `memcmp` always returns an `i32` regardless of the platform.  I'm running into some issues and was hoping I could get some help.

Here's what I've been attempting so far:

1. Build the stage0 compiler with all the changes _expect_ for the changes in `library/core/src/slice/cmp.rs` and `compiler/rustc_codegen_llvm/src/context.rs`; this is because `target_c_int_width` isn't passed through and recognized as a valid config option yet.  I'm building with `./x.py build --stage 0 library/core library/proc_macro compiler/rustc`
2. Next I add in the `#[cfg(c_int_width = ...)]` params to `cmp.rs` and `context.rs` and build the stage 1 compiler by running `./x.py build --keep-stage 0 --stage 1 library/core library/proc_macro compiler/rustc`.  This step now runs successfully.
3. Lastly, I try to build the test program for AVR mentioned in #78022 with `RUSTFLAGS="--emit llvm-ir" cargo build --release`, and look at the resulting llvm IR, which still shows:

```
...
%11 = call addrspace(1) i32 `@memcmp(i8*` nonnull %5, i8* nonnull %10, i16 5) #7, !dbg !1191                                                                                                                                                                                                                                %.not = icmp eq i32 %11, 0, !dbg !1191
...
; Function Attrs: nounwind optsize                                                                                                                                                                                                                                                                                          declare i32 `@memcmp(i8*,` i8*, i16) local_unnamed_addr addrspace(1) #4
```

Any ideas what I'm missing here?  Alternately, if this is totally the wrong approach I'm open to other suggestions.

cc `@Rahix`

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