gdbstub/helpers: Replace TARGET_BIG_ENDIAN -> target_big_endian()

gdbstub/helpers: Replace TARGET_BIG_ENDIAN -> target_big_endian()

Check endianness at runtime to remove the target-specific
TARGET_BIG_ENDIAN definition. Use cpu_to_[be,le]XX() from
"qemu/bswap.h" instead of tswapXX() from "exec/tswap.h".

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250708215320.70426-7-philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>