gdb: LoongArch: Refactor member functions of loongarch_frame_unwind

gdb: LoongArch: Refactor member functions of loongarch_frame_unwind

In the current code, loongarch_frame_unwind is a LoongArch prologue unwinder,
it contains the required member functions, but they do not calculate a valid
frame id through prologue of a function frame. Refactor these functions and
use loongarch_frame_cache to record the information of the function frame.
No functional change intended.

Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>