hw/intc/loongarch_pch_pic: Fix ubsan warning and endianness issue

hw/intc/loongarch_pch_pic: Fix ubsan warning and endianness issue

When booting the Linux kernel from tests/functional/test_loongarch64_virt.py
with a QEMU that has been compiled with --enable-ubsan, there is
a warning like this:

 .../hw/intc/loongarch_pch_pic.c:171:46: runtime error: index 512 out of
  bounds for type 'uint8_t[64]' (aka 'unsigned char[64]')
 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
  .../hw/intc/loongarch_pch_pic.c:171:46
 .../hw/intc/loongarch_pch_pic.c:175:45: runtime error: index 256 out of
  bounds for type 'uint8_t[64]' (aka 'unsigned char[64]')
 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
  .../hw/intc/loongarch_pch_pic.c:175:45

It happens because "addr" is added first before substracting the base
(PCH_PIC_HTMSI_VEC or PCH_PIC_ROUTE_ENTRY).
Additionally, this code looks like it is not endianness safe, since
it uses a 64-bit pointer to write values into an array of 8-bit values.

Thus rework the code to use the stq_le_p / ldq_le_p helpers here
and make sure that we do not create pointers with undefined behavior
by accident.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>