Make the 32-bit ARM get_user() and put_user() work for 16-bit quantities

Make the 32-bit ARM get_user() and put_user() work for 16-bit quantities

The 32-bit ARM does not have instructions to perform 16-bit loads or
stores.  The __get_user_asm_half and __put_user_asm_half macros
sythesize those operations.  However, in most cases the pointers
passed into these macros are pointers to 16-bit types and the pointer
arithmetic will end up pointing at the next 16-bit quantity rather
than the second half (byte) of the 16-bit quantity.  The macros need
to explicitly typecast the pointers as pointers to 8-bit quantities to
make the pointer arithmetic work out properly.