Prederefed run cores

With the indirect register addressing all prederefed run cores 
(Prederefed, CGP, Switch) are currently not functional, as these run 
cores have absolute addresses in the prederefed code.

I see two ways to fix it:

1) use frame pointer relative addressing:
   + prederefed code is usable by different threads too
   - ~4 times increase in code size of core_ops_*.{c,o} [1]

2) Re-prederef on function calls, if frame pointer differs
   + no impact on code size
   - needs precise code length of functions
   - threads need distinct prederefed code
   - possibly slower then 1)

Comments welcome,
leo

[1] due to absolute addressing a constant argument and a register 
argument have the same code, set_i_ic and set_i_i are the same.