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>

hw/intc/loongarch_pch_pic.c

@@ -110,10 +110,10 @@ static uint64_t pch_pic_read(void *opaque, hwaddr addr, uint64_t field_mask)
         val = s->int_polarity;
         break;
     case PCH_PIC_HTMSI_VEC ... PCH_PIC_HTMSI_VEC_END:
-        val = *(uint64_t *)(s->htmsi_vector + addr - PCH_PIC_HTMSI_VEC);
+        val = ldq_le_p(&s->htmsi_vector[addr - PCH_PIC_HTMSI_VEC]);
         break;
     case PCH_PIC_ROUTE_ENTRY ... PCH_PIC_ROUTE_ENTRY_END:
-        val = *(uint64_t *)(s->route_entry + addr - PCH_PIC_ROUTE_ENTRY);
+        val = ldq_le_p(&s->route_entry[addr - PCH_PIC_ROUTE_ENTRY]);
         break;
     default:
         qemu_log_mask(LOG_GUEST_ERROR,
@@ -129,7 +129,8 @@ static void pch_pic_write(void *opaque, hwaddr addr, uint64_t value,
 {
     LoongArchPICCommonState *s = LOONGARCH_PIC_COMMON(opaque);
     uint32_t offset;
-    uint64_t old, mask, data, *ptemp;
+    uint64_t old, mask, data;
+    void *ptemp;
 
     offset = addr & 7;
     addr -= offset;
@@ -168,12 +169,12 @@ static void pch_pic_write(void *opaque, hwaddr addr, uint64_t value,
         s->int_polarity = (s->int_polarity & ~mask) | data;
         break;
     case PCH_PIC_HTMSI_VEC ... PCH_PIC_HTMSI_VEC_END:
-        ptemp = (uint64_t *)(s->htmsi_vector + addr - PCH_PIC_HTMSI_VEC);
-        *ptemp = (*ptemp & ~mask) | data;
+        ptemp = &s->htmsi_vector[addr - PCH_PIC_HTMSI_VEC];
+        stq_le_p(ptemp, (ldq_le_p(ptemp) & ~mask) | data);
         break;
     case PCH_PIC_ROUTE_ENTRY ... PCH_PIC_ROUTE_ENTRY_END:
-        ptemp = (uint64_t *)(s->route_entry + addr - PCH_PIC_ROUTE_ENTRY);
-        *ptemp = (*ptemp & ~mask) | data;
+        ptemp = (uint64_t *)&s->route_entry[addr - PCH_PIC_ROUTE_ENTRY];
+        stq_le_p(ptemp, (ldq_le_p(ptemp) & ~mask) | data);
         break;
     default:
         qemu_log_mask(LOG_GUEST_ERROR,