build: Checking for compiler support of LoongArch LSX should be guarded

In the configure script, checking whether the LoongArch LSX intrinsics
are supported by the compiler was done unconditionally, regardless of
the targetted host platform. Compared to how we support the other SIMD
platforms and compilers, this is rather unconventional.

We are placing this check under the guard of its own platform, for the
time being. A full solution, in line with the rest of the configure.ac
patterns concering SIMD optimizations, is TODO.

We also do an overall cleanup in the SIMD section of configure.ac, and,
finally, we regenerate the configure script.