powerpc: Streamline ret_from_except_lite for non-iSeries platforms

powerpc: Streamline ret_from_except_lite for non-iSeries platforms

There is a small passage of code in ret_from_except_lite which is
only required on iSeries.  For a multi-platform kernel on non-iSeries
machines this means we end up executing ~15 nops in ret_from_except_lite.

It would be nicer if non-iSeries could skip the code entirely, and on
iSeries we can jump out of line to execute the code.

I have no performance numbers to justify this, other than the assertion
that executing 15 nops takes longer than executing 0.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>