LoongArch: ftrace: Add direct call trampoline samples support

The ftrace samples need per-architecture trampoline implementations to
save and restore argument registers around the calls to my_direct_func*
and to restore polluted registers (e.g: ra).

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