target/arm: Fix handling of setting SVE registers from gdb

target/arm: Fix handling of setting SVE registers from gdb

The code to handle setting SVE registers via the gdbstub is broken:
 * it sets each pair of elements in the zregs[].d[] array in the
   wrong order for the most common (little endian) case: the least
   significant 64-bit value comes first
 * it makes no attempt to handle target_endian()
 * it does a simple copy out of the (target endian) gdbstub buffer
   into the (host endan) zregs data structure, which is wrong on
   big endian hosts

Fix all these problems:
 * use ldq_p() to read from the gdbstub buffer
 * check target_big_endian() to see if we need to handle the
   128-bit values the opposite way around

Cc: qemu-stable@nongnu.org
Signed-off-by: Vacha Bhavsar <vacha.bhavsar@oss.qualcomm.com>
Message-id: 20250722173736.2332529-3-vacha.bhavsar@oss.qualcomm.com
[PMM: adjusted commit message, fixed spacing]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>