linux-user: Use correct target SHMLBA in shmat()

The shmat() handling needs to do target-specific handling
of the attach address for shmat():
 * if the SHM_RND flag is passed, the address is rounded
   down to a SHMLBA boundary
 * if SHM_RND is not passed, then the call is failed EINVAL
   if the address is not a multiple of SHMLBA

Since SHMLBA is target-specific, we need to do this
checking and rounding in QEMU and can't leave it up to the
host syscall.

Allow targets to define TARGET_FORCE_SHMLBA and provide
a target_shmlba() function if appropriate, and update
do_shmat() to honour them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
