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

[WÁNG Xuěruì via llvm-commits]

https://github.com/xen0n commented:

It's nice to see vectorization being leveraged across the board, as per the recommended baseline!

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.

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