arch/arm: add THUMB interworking support

Add config option to build wtarget code with THUMB interworking.
This is used to build the C library as well as all other code
that runs on the target.

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }