Report rvv bug (missing vsetvli) in upstream gcc and remove workaround

[PhilippvK]

According to the RVV spec, it the job of the compiler (LLVM, GCC) to insert appropriate vsetvli if vtype changes between operations: https://github.com/riscv-non-isa/rvv-intrinsic-doc/blob/c783839de5a59480d09f1a9b69257fca4096434e/archive/rvv-intrinsic-rfc.md#sew-and-lmul-of-intrinsics

LLVM seems to do this all the time, but the version of the RISC-V gcc we use (13.2) seems to be generating invalid programs sometimes, causing spike to crash.

We had to make the vl volatile here to prevent GCC from breaking this kernel.
See here:

muriscv-nn/Source/BasicMathFunctions/muriscv_nn_elementwise_mul_s16.c

Line 67 in b6e3d83

volatile size_t vl = vsetvl_e16m4(loop_count);

We should figure it if this got already patched upstream and whether the workaround can be dropped. If we need to keep it, we should remove the volatile and instead add the missing vsetvli in an #if __GNUC__ >= 13...