exec: Move [b]tswapl() declarations to 'exec/user/tswap-target.h'

exec: Move [b]tswapl() declarations to 'exec/user/tswap-target.h'

tswapl() and bswaptls() are target-dependent and only used
by user emulation. Move their definitions to a new header:
"exec/user/tswap-target.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20231212123401.37493-17-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>