Var-tracking generates broken DW_AT_frame_base on s390

[Josef Zlomek]

> > I see. The combination of location list for frame base and
> > the location expr (orig debug info) for a variable makes the problem.
> 
> Exactly.  Do you have any suggestions how to fix this?

I have a patch which changes the fixed point from the stack pointer at
the beginning of function to the stack pointer after prologue.
I tested it on s390 and x86-64 and it worked well.
It is not bootstrapped/regtested yet, I'll send it later.

Josef