selftests: Test RISC-V Vector's first-use handler

This add a test to check if the kernel zero-initializes all V registers
after the first-use trap handler returns.

If V registers are not zero-initialized, then the test should fail one
out of several runs:

```
 root@sifive-fpga:~# ./v_initval_nolibc
 # vl = 256
 not ok 1 detect stale values on v-regesters
 0 0 0 0 0 0 0 0   0 0 0 0 0 0 0 0
 0 4c 41 4e 47 3d 43 0   50 41 54 48 3d 2f 75 73
 72 2f 6c 6f 63 61 6c 2f   73 62 69 6e 3a 2f 75 73
 72 2f 6c 6f 63 61 6c 2f   62 69 6e 3a 2f 75 73 72
 ff ff 81 0 0 0 0 0   0 0 0 0 0 0 0 0
```

Otherwise, the test passes without errors each run.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230627015556.12329-3-andy.chiu@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>