iw: change __cmd section scheme to fit gcc/clang

iw: change __cmd section scheme to fit gcc/clang

There are some behaviour differences in how gcc and clang
fill a special section like the one we use for __cmd here.

In both compilers, obviously all variables declared like
this end up in the special __cmd section.

However, alignment and padding is applied differently by
different compilers and linkers, and as such it is hard to
really rely on being able to treat this like an array.

To avoid all these issues, put just *pointers* to the
structures into the special section, which makes it easy
to skip NULL pointers (resulting from padding) and has no
issues with alignment as they're always smaller than the
alignment applied.

Change-Id: I35e2c66e541758e60363883d26b31d2040ce8aa9
Signed-off-by: Johannes Berg <johannes.berg@intel.com>