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>

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_string.tc

@@ -28,6 +28,9 @@ s390*)
 mips*)
   ARG1=%r4
 ;;
+loongarch*)
+  ARG1=%r4
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_syntax.tc

@@ -40,6 +40,10 @@ mips*)
   GOODREG=%r4
   BADREG=%r12
 ;;
+loongarch*)
+  GOODREG=%r4
+  BADREG=%r12
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested