userspace: align _k_object to 4 bytes

We want the struct to be packed to conserve space, but the
perms field needs to always be on a 4-byte boundary since
we do bitfield operations on it, arches like ARC require
that the sys_bitfield_* operations be aligned to a 4 byte
boundary.

Instances of struct _k_object will now be 4-byte aligned
if in an array (which they are), even though the members
are still packed.

Fixes: #7776

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>