[POWERPC] 64bit FPSCR support

Forthcoming machines will extend the FPSCR to 64 bits.  We already
had a 64-bit save area for the FPSCR, but we need to use a new form
of the mtfsf instruction.  Fortunately this new form is decoded as
an ordinary mtfsf by existing 64-bit processors.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>