linux-user: register align p{read, write}64

linux-user: register align p{read, write}64

pread64 and pwrite64 pass 64bit parameters which for some architectures need
to be aligned to special argument pairs, creating a gap argument.

Handle this special case the same way we handle it in other places of the code.

Reported-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Alex Barcelo <abarcelo@ac.upc.edu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>