The m68k port was using a one-to-one mapping between dwarf register numbers and 
gdb register numbers.  That's incorrect as the floating point registers have 
dwarf numbers 16..23 but gdb register numbers 18..25.  Furthermore, GCC 
indicates the return column is register 25, which maps onto the last fp register.

This patch adds a mapping function.  ok?

nathan
-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery
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