selftests/ftrace: Add LoongArch kprobe args string tests support

Before:
[5] Kprobe event string type argument	[UNTESTED]
[7] Kprobe event argument syntax	[UNTESTED]

After:
[5] Kprobe event string type argument	[PASS]
[7] Kprobe event argument syntax	[PASS]

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>