I tried many devices which mentionned in their description that they are supporting thumb extension or devices with a "J" in the architecture name, such as NXP LPC3180 (ARM926EJ-S based high-performance 32-bit RISC Microcontroller with Thumb extensions).
When I specify in the code 'THUMBX' I get:
TMS470R1.s(1091): error: A1461E: Specified processor or architecture does not support Thumb instructions
Martin