[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.

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