[llvm] [LoongArch] Enable FeatureExtLSX for generic-la64 processor (PR #113421)

[via llvm-commits]

Ami-zhang wrote:

> Note to myself: While this clearly affects `-march=generic` for Clang, I haven't verified whether `-march=loongarch64` behavior stays the same as GCC (no LSX).
> 
> 2nd note to myself: GCC needs `-march=generic` implemented too for feature parity.

Thanks for your thoughtfulness and for sharing your insights! Of course, We will also continue to take note of matters related to `-march=generic`.

https://github.com/llvm/llvm-project/pull/113421