Fixes two errors produced on except.S by the IAR assembler (as seen in
issue #13200).

The first: "'ALIGN' is illegal. Use directive ALIGNROM or ALIGNRAM (the
first parameter is raised to the power of 2, the second is a 32-bit fill
value)."
Fixed by changing ALIGN to ALIGNROM 2 to align to a 4-byte boundary
(mirroring what ALIGN does in the ARM except.S).

The second (on THUMB): "Must be placed inside a section"
Fixed by moving THUMB after the SECTION directive so that it affects
that section.