[clang] [flang] [mlir] [flang] Retry add basic -mtune support (PR #96688)

[Andrzej Warzyński via cfe-commits]

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+; RUN: mlir-translate -import-llvm -split-input-file %s | FileCheck %s
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banach-space wrote:

I have X86 disabled - I just double checked. And yes, this test works for me just fine without the guard. Alexis, what error do you get when it fails for you? 

To me, it would make sense if `mlir-translate` didn't care - it doesn't have to interpret these attributes, does it? 

Btw @AlexisPerry , there's no need to clone "tune-cpu.f90" as [tune-cpu-llvm-aarch.f90](https://github.com/llvm/llvm-project/pull/96688/files#diff-879569867a123346813864ce36bc15249acb8b57bc335c808eb4d30818ec8685). Instead, you can use conditional RUN lines like here: 
* https://github.com/llvm/llvm-project/blob/main/flang/test/Driver/pic-flags.f90

https://github.com/llvm/llvm-project/pull/96688