linux-user: correct how SOL_SOCKET is converted from target to host and back

Previous implementation does not take into account that SOL_SOCKET constant
can be arch specific. This change fixes some issues with sendmsg/recvmsg.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>