gdb: LoongArch: Adjust loongarch_stopped_data_address()

loongarch_stopped_data_address() is a common function and will be used by
gdb and gdbserver, so move its definition from gdb/loongarch-linux-nat.c
to gdb/nat/loongarch-hw-point.c. This is preparation for later gdbserver
patch on LoongArch and is no effect for the current code.

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