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.

 reporter_name="Julien DUSSER"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00054.html"

 reporter_comment="Soft-float for the 405 PowerPC"