Rollup merge of #145309 - winstonallo:issue-145271-fix, r=tgross35

Fix `-Zregparm` for LLVM builtins

This fixes the issue where `-Zregparm=N` was not working correctly when calling LLVM intrinsics

By default on `x86-32`, arguments are passed on the stack. The `-Zregparm=N` flag allows the first `N` arguments to be passed in registers instead.

When calling intrinsics like `memset`, LLVM still passes parameters on the stack, which prevents optimizations like tail calls.

As proposed by ````@tgross35,```` I fixed this by setting the `NumRegisterParameters` LLVM module flag to `N` when the `-Zregparm=N` is set.

```rust
// compiler/rust_codegen_llvm/src/context.rs#375-382
if let Some(regparm_count) = sess.opts.unstable_opts.regparm {
    llvm::add_module_flag_u32(
        llmod,
        llvm::ModuleFlagMergeBehavior::Error,
        "NumRegisterParameters",
        regparm_count,
    );
}
```
[Here](https://rust.godbolt.org/z/YMezreo48) is a before/after compiler explorer.

Here is the final result for the code snippet in the original issue:
```asm
entrypoint:
        push    esi
        mov     esi, eax
        mov     eax, ecx
        mov     ecx, esi
        pop     esi
        jmp     memset   ; Tail call parameters in registers
```

Fixes: https://github.com/rust-lang/rust/issues/145271

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