IAR: Fix building with IAR ARM 6.X
This PR fixes a regression from !3196 (merged). Unfortunately this slipped though my original tests.
During development of this PR I have tested IAR ARM 4.0 5.4 6.4 and 6.5
Since 4.0 is based one a different linker model it is currently unsupported. I have placed a FATAL_ERROR to exit early instead of fiddling wit obscure error messages, what has just happened to me. Theoretically it should be possible to support even this old compiler, but it will add a lot of extra conditions. For example it is detected as IAR ARM 0.0.441.