Fix IAR assembler errors on except.S

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.

platform/source/TARGET_CORTEX_M/TOOLCHAIN_IAR/except.S

@@ -36,9 +36,9 @@ FAULT_TYPE_BUS_FAULT            EQU     0x30
 FAULT_TYPE_USAGE_FAULT          EQU     0x40
 
         PRESERVE8
-        THUMB
 
         SECTION .text:CODE:NOROOT(2)
+        THUMB
 
 HardFault_Handler
         EXPORT  HardFault_Handler
@@ -111,7 +111,7 @@ Fault_Handler_Continue2
 #else
         B       .
 #endif
-        ALIGN
+        ALIGNROM 2
 #endif
 
         END