toolchain: linker: IAR fix for new linker optimization options

The IAR linker ilink does not have any optimization options, and
with the newly added linker optimization flags that default to
the compiler optimization flags, ilink will fail with an
"illegal command line option" error.
This temporary workaround will use a harmless option instead to
avoid errors, until we can update the linker.

Signed-off-by: Lars-Ove Karlsson <lars-ove.karlsson@iar.com>