powerpc: Fix STAB initialization

Commit f5339277 accidentally removed
more than just iSeries bits and took out the call to stab_initialize()
thus breaking support for POWER3 processors.

Put it back. (Yes, nobody noticed until now ...)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org> [v3.4+]