Change .fef8 and .fif8 to pass values on the stack.

Change .fef8 and .fif8 to pass values on the stack.

Reorder the code in .fef8 and .fif8 so that in the usual case, we fall
through to the blr without taking any branches.  The usual case, by my
guess, is .fef8 with normalized numbers or .fif8 with small integers.

I change .fef8 and .fif8 to pass values on the real stack, not in
specific registers.  This simplifies the ncg table, and might help me
experiment with changes to the ncg table.

This change might or might not help mcg.  Seems that mcg always uses
the stack to pass values to libem, but I have not tested .fef8 or
.fif8 with mcg.