Revert "convert SLB miss handlers to C" and subsequent commits

This reverts commits:
  5e46e29e ("powerpc/64s/hash: convert SLB miss handlers to C")
  8fed04d0 ("powerpc/64s/hash: remove user SLB data from the paca")
  655deecf ("powerpc/64s/hash: SLB allocation status bitmaps")
  2e162674 ("powerpc/64s/hash: provide arch_setup_exec hooks for hash slice setup")
  89ca4e12 ("powerpc/64s/hash: Add a SLB preload cache")

This series had a few bugs, and the fixes are not all trivial. So
revert most of it for now.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>