linux-user: added support for preadv() system call.

linux-user: added support for preadv() system call.

This system call performs the same task as the readv() system call,
with the exception of having the fourth argument, offset, which
specifes the file offset at which the input operation is to be performed.
Because of this, the preadv() implementation is based on the readv()
implementation in linux-user mode.

But, since preadv() is implemented in the kernel as a 5-argument syscall,
5 arguments are needed to be handled as input and passed to the host
syscall.

The pos_l and pos_h argument of the safe_preadv() are of type unsigned
long, which can be of different sizes on different platforms. The input
arguments are converted to the appropriate host size when passed to
safe_preadv().

Signed-off-by: Dejan Jovicevic <dejan.jovicevic@rt-rk.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>