powerpc: make additional room in exception vector area

The FWNMI region is fixed at 0x7000 and the vector are now overflowing
that with allmodconfig. Fix that by moving slb_miss_realmode code out
of that region as it doesn't need to be that close to the call sites
(it is a _GLOBAL function)

Fixes this build error:

arch/powerpc/kernel/exceptions-64s.S: Assembler messages:
arch/powerpc/kernel/exceptions-64s.S:1304: Error: attempt to move .org backwards

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>