iar: linker_script: fix bug with alphabetical order

Having alphabetical order on an init block causes a linker error.
So it is one of the things replaced when translating a block to its
corresponding init block.
The recent changes in ff8b24b6 caused
iterable sections having ONLY alphabetical order as an attribute,
triggering a bug in this regexp.

Signed-off-by: Robin Kastberg <robin.kastberg@iar.com>