Darwin has HTON*/NTOH* macros that on BE simply return the argument.
This is incompatible with SLIRP's use of these macros as a statement.

Undefine the macros in the HOST_WORDS_BIGENDIAN code path to redefine
these macros as no-op, as already done when they were undefined.
Suggested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>