LoongArch: Adjust PC value when unwind next frame in unwinder

When state->first is not set, the PC is a return address in the previous
frame. We need to adjust its value in case overflow to the next symbol.

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