powerpc/pseries: Simplify denormalization handler

The following simplifies the denorm code by using macros to generate the long
stream of almost identical instructions.

This patch results in no changes to the output binary, but removes a lot of
lines of code.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>