dynamic PIC register?

[dalej@apple.com]

On the Darwin powerpc target, it would be better not to have 
PIC_OFFSET_TABLE_REGNUM be a fixed register.  In particular, it should be 
a register saved over calls in a non-leaf function, and a register not 
saved over calls in a leaf function.  I could  do this by changing the 
fixed register at the last minute in the target-dependent prolog/epilog 
generation (which is what we did in the 2.95 version currently shipping).  
But it seems to me a cleaner solution that also applies to other targets 
might be useful.  Is this so, and how might I go about it?