[rfc/rft] Eliminate gdbarch_read_sp callback function

[Joel Brobecker]

> The avr-tdep.c and score-tdep.c conversions are straightfoward; but I'm
> not completely sure about the mips-tdep.c.  The old read_sp implementation
> used to read the real MIPS_SP_REGNUM, but the unwind_pc implementation
> unwinds the pseudo NUM_REGS + mips_regnum (gdbarch)->pc.

FWIW: I tested your patch on mips-irix, and it showed no difference
in results.

-- 
Joel