riscv: Add kprobes KUnit test

riscv: Add kprobes KUnit test

Add KUnit test for riscv kprobes, mostly for simulated instructions. The
test install kprobes into multiple sample functions, and check that these
functions still return the expected magic value.

This test can detect some kprobe bugs reported in the past (in Link:).

Link: https://lore.kernel.org/linux-riscv/20241119111056.2554419-1-namcao@linutronix.de/
Link: https://lore.kernel.org/stable/c7e463c0-8cad-4f4e-addd-195c06b7b6de@iscas.ac.cn/
Link: https://lore.kernel.org/linux-riscv/20230829182500.61875-1-namcaov@gmail.com/
Signed-off-by: Nam Cao <namcao@linutronix.de>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250513151631.3520793-1-namcao@linutronix.de
Signed-off-by: Paul Walmsley <pjw@kernel.org>