target/arm: Fix big-endian handling of NEON gdb remote debugging

In the code for allowing the gdbstub to set the value of an AArch64
FP/SIMD register, we weren't accounting for target_big_endian()
being true. This meant that for aarch64_be-linux-user we would
set the two halves of the FP register the wrong way around.
The much more common case of a little-endian guest is not affected;
nor are big-endian hosts.

Correct the handling of this case.

Cc: qemu-stable@nongnu.org
Signed-off-by: Vacha Bhavsar <vacha.bhavsar@oss.qualcomm.com>
Message-id: 20250722173736.2332529-2-vacha.bhavsar@oss.qualcomm.com
[PMM: added comment, expanded commit message, fixed missing space]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/gdbstub64.c

@@ -115,8 +115,22 @@ int aarch64_gdb_set_fpu_reg(CPUState *cs, uint8_t *buf, int reg)
         /* 128 bit FP register */
         {
             uint64_t *q = aa64_vfp_qreg(env, reg);
-            q[0] = ldq_le_p(buf);
-            q[1] = ldq_le_p(buf + 8);
+
+            /*
+             * On the wire these are target-endian 128 bit values.
+             * In the CPU state these are host-order uint64_t values
+             * with the least-significant one first. This means they're
+             * the other way around for target_big_endian() (which is
+             * only true for us for aarch64_be-linux-user).
+             */
+            if (target_big_endian()) {
+                q[1] = ldq_p(buf);
+                q[0] = ldq_p(buf + 8);
+            } else{
+                q[0] = ldq_p(buf);
+                q[1] = ldq_p(buf + 8);
+            }
+
             return 16;
         }
     case 32: