fix RVV 1.0 detection code

[markdryan]

There were a couple of issues with the detection code used to check for RVV 1.0 on kernels that do not support hwprobe.

1. The vtype clobber was missing
2. The wrong form of vsetvli was being used. The vsetvli x0, x0 form is inappropriate for this use case as it can only be safely used in code where the value of vtype is known. The use of vsetvli x0, x0 here can lead to a failure to detect RVV 1.0, if, for example, the vill bit happens to be set before detect_riscv64_rvv100 is called.

We fix both issues by adding the missing clobber and replacing the first parameter to vsetvli with t0 (which we add to our clobbers).

[markdryan]

I discovered this when working on #5431. When testing that PR I noticed that the RVV kernels were not always enabled on devices that supported RVV 1.0 but which were running kernels without hwprobe.

[martin-frbg]

The failing DYNAMIC_ARCH job has

   71 |                      :"t0", "vtype");

probably because it is running a riscv-gnu-clang toolchain from a year ago. This probably explains the previously missing clobber - I'm not sure if we should simply require users to have a newer one ?