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

[Lu Weining via llvm-commits]

SixWeining wrote:

> This patch enables LSX by default in Clang, whereas GCC does not. Should we align the behavior with GCC?

Actually it has been enabled in #100056.

Yes, clang and gcc have different mechanisms to enable lsx by default. Clang achieves this by how its source code is written while gcc achieves it by how it is configured when being built.

AFAIK, the upcoming [soft-dev-conv](https://github.com/loongson/la-softdev-convention) requires developers must enable LSX by default when building desktop and server operating systems (but not for embedded systems). Maybe maintainer of linux distribution will turn on LSX when they configure GCC.

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