Patch to add missing PPC instruction

[David Edelsohn]

>>>>> degger  writes:

degger> A candidate for -ffast-math? I originally meant it that way but forgot
degger> about it between the time I wrote the patch and sent it here 
degger> (>4 months).

degger> For some applications (those who are using single precision arithmetics)
degger> it might actually be enough to such an unaccurate reciprocal but I
degger> definitely see your point when you say it's not acceptable to be used in 
degger> general.

	One part in 256 is too fast and loose for -ffast-math, IMHO.  It
is meant as a graphics assist instruction.

	If the programmer thinks this is useful, the programmer should use
the instruction directly; the compiler should not be substituting this
instruction except as part of a Newton-Raphson approximation to the
required precision.

David