[Bug target/112817] RISC-V: RVV: provide a preprocessor macro for VLS codegen

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112817

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note the reality is xsimd was not thought out for SIMD but rather just fixed
length extensions. It seems more like a major shift that needs to happen to
these libraries and stop just thinking fixed length registers.