Fix ARM EABI profiling

[Paul Brook]

> R12 is only clobbered with ARM->Thumb linker glue, so if our mcount is
> built in ARM mode (unless building for non-interworking Thumb, in which
> case no glue is generated) then I don't believe there is a problem.

Having the Application and _mount implementation both in the same mode is a 
fairly big assumption. It violates the EABI requirement that code be 
interworking safe.

Paul