[llvm] Introduce MCAsmInfo::UsesSetToEquateSymbol and prefer = to .set (PR #142289)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> @MaskRay A lit test failed on [lab.llvm.org/buildbot#/builders/64/builds/4195](https://lab.llvm.org/buildbot/#/builders/64/builds/4195), are you able to put in a quick fix?
> 
> ```
> Input file: <stdin>
> Check file: /home/llvm/llvm-external-buildbots/workers/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/MC/AsmParser/include.ll
> -dump-input=help explains the following input dump.
> Input was:
> <<<<<<
>             1:  .file "/home/llvm/llvm-external-buildbots/workers/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/test/MC/AsmParser/include.ll",,"LLVM version 21.0.0git" 
> check:13'0     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
>             2:  .csect ..text..[PR],5 
> check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~
>             3:  .rename ..text..[PR],"" 
> check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~
>             4:  # Start of file scope inline assembly 
> check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>             5:  
> check:13'0     ~
>             6:  
> check:13'0     ~
>             7: .set MODULE, 1 
> check:13'0     ~~~~~~~~~~~~~~~
> check:13'1          ?          possible intended match
>             8:  
> check:13'0     ~
>             9:  
> check:13'0     ~
>            10:  
> check:13'0     ~
>            11:  # End of file scope inline assembly 
> check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>            12:  .machine "PWR7" 
> check:13'0     ~~~~~~~~~~~~~~~~~
>             .
>             .
>             .
> >>>>>>
> --
> ********************
> ```

Looks like llvm/test/MC/AsmParser/include.ll should have an explicit target triple to avoid `UNSUPPORTED`... Specified an explicit triple for it and it should work on all hosts that use `.set`

https://github.com/llvm/llvm-project/pull/142289