gdb: LoongArch: Adjust loongarch_{get,remove}_debug_reg_state()

loongarch_{get,remove}_debug_reg_state() are used as helper functions
by loongarch_linux_nat_target. We should move their definitions from
gdb/nat/loongarch-linux-hw-point.c to gdb/loongarch-linux-nat.c.

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