LoongArch: Get frame info in unwind_start() when regs is not available

At unwind_start(), it is better to get its frame info here rather than
get them outside, even we don't have 'regs'. In this way we can simply
use unwind_{start, next_frame, done} outside.

Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>