[llvm] [Exegesis] CPU selection, when native arch and target mismatch (PR #131014)

[via llvm-commits]

AnastasiyaChernikova wrote:

> > At the moment, when building llvm-exegesis for RISC-V, the --mcpu flag must be specified. If this flag is omitted, the mcpu of the architecture of the device on which the assembly takes place (often ARM or x86) will be automatically set. To address this issue, I suggest making the following changes: if the launch line does not contain the mcpu when building for a different architecture, the first one from the list of mcpu for the target architecture will be automatically selected.
> 
> Is the first CPU in the list always a sensible choice? I think it would make more sense to just error out if the user species `-mcpu=native` when the target arch is set to something different than the host.

In this case, we are talking about a situation where the user does not set any mcpu. In this scenario, exegesis itself puts the mcpu in native. And already during the processing process, the basic mcpu of the architecture on which the assembly is taking place is substituted. I believe that when the mcpu is not set by the user, it is assumed that just the basic one will be used, as it is done in the case of non-cross compilation. That's the functionality I'm offering here.
Regarding the first item from the list - I agree. Replaced with the choice of generic.

https://github.com/llvm/llvm-project/pull/131014