powerpc: Make room in exception vector area

The FWNMI region is fixed at 0x7000 and the vector are now
overflowing that with some configurations. Fix that by moving
some hash management code out of that region as it doesn't need
to be that close to the call sites (isn't accessed using
conditional branches).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>