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>