tools: selftests: riscv: Fix compile warnings in vector tests

GCC prints a couple of format string warnings when compiling
the vector tests. Let's follow the recommendation in
Documentation/printk-formats.txt to fix these warnings.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231123185821.2272504-5-christoph.muellner@vrull.eu
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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>