fix RVV 1.0 detection code

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).

driver/others/detect_riscv64.c

@@ -63,12 +63,12 @@ uint64_t detect_riscv64_rvv100(void)
 	 * RVV 1.0 and we return 0.
 	 */
 
-	asm volatile("vsetvli x0, x0, e8, m1, ta, ma\n\t"
+	asm volatile("vsetvli t0, x0, e8, m1, ta, ma\n\t"
 		     "csrr %0, vtype\n\t"
 		     "slt %0, x0, %0\n"
 		     : "=r" (rvv10_supported)
 		     :
-		     :);
+		     :"t0", "vtype");
 
 	return rvv10_supported;
 }