Hello,

This patch moves the two identical versions of skip_prologue_using_sal, 
found in both mips-tdep and frv-tdep, into symtab.c (just after 
in_prologue).

I've also added comments to the two refine_prologue_limit variants 
(found in ia64-tdep and rs6000-tdep) pointing at the new code.

comments? symtab ok?
Andrew