linux-user: add TARGET_SO_{DOMAIN,PROTOCOL}

These were defined for other platforms but mistakenly left out of mips
and generic, so this commit adds them to the places missing. Then it
makes them be translated in getsockopt.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210204153925.2030606-1-Jason@zx2c4.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>