linux-user: Restore cast to target type in get_user()

Commit 658f2dc9 accidentally dropped the cast to the target type of
the value loaded by get_user().  The most visible effect of this would
be that the sequence "uint64_t v; get_user_u32(v, addr)" would sign
extend the 32 bit loaded value into v rather than zero extending as
would be expected for a _u32 accessor.  Put the cast back again to
restore the old behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>