exec: Declare target_words_bigendian() in 'exec/tswap.h'

exec: Declare target_words_bigendian() in 'exec/tswap.h'

We usually check target endianess before swapping values,
so target_words_bigendian() declaration makes sense in
"exec/tswap.h" with the target swapping helpers.

Remove "hw/core/cpu.h" when it was only included to get
the target_words_bigendian() declaration.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20231212123401.37493-16-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>