RISC-V: exploit `-mrvv-vector-bits=zvl` when used

Here CONFIG=1 defaults to a run-time determination of the hardware vector length:

Line 20 in cf532e2

#define SLEEF_RVV_VLEN __riscv_vlenb()

If the symbol __riscv_v_fixed_vlen is defined then the compiler is generating code which supports only that specified vector length (measured in bits), and we can lean into this by using the compile-time macro instead of the intrinsic call.

I have patches outstanding which edit this code, but they don't implement this suggestion. To make the change requires a bunch of extra CI permutations to test with and without -mrvv-vector-bits=zvl so it's not as trivial as it sounds.

CC: @luhenry

I don't expect a drastic performance gain, but it might help a little.

-mrvv-vector-bits= should be the exception especially for the distribution of a binary/library as it would then be tied to whatever it was compiled for. Unless it's a functional requirement, or it's known that the binary is going to target a single class/model of hardware, then it's more pain than gain.