support loongson LASX runtime dispatching on LLVM

Typically, under loongson systems, the library will be compiled with LSX builtin, but not LASX even if it is supported at the CPU level. Under GCC 15+, we support runtime dispatching: LSX and LASX are both built, and we determine the best kernel at runtime. This is not currently enabled when compiled with LLVM.

https://godbolt.org/z/G1PasdGGa