Reintroduce -mflat option on SPARC

[Eric Botcazou]

> Another question: why does the model hijack %i7 to use it as frame pointer,
> instead of just using %fp?  AFAICS both are kept as fixed registers by the
> code so the model seems to be wasting 1 register (2 without frame pointer).

Related question: why saving the Local and In registers in the frame instead of 
at their standard location, right above the stack pointer?  It would seem to 
me that the layout of the frame can be identical to the standard one.

-- 
Eric Botcazou