Add SLB switching code for entry/exit

This is the really low level of guest entry/exit code.

Book3s_64 has an SLB, which stores all ESID -> VSID mappings we're
currently aware of.

The segments in the guest differ from the ones on the host, so we need
to switch the SLB to tell the MMU that we're in a new context.

So we store a shadow of the guest's SLB in the PACA, switch to that on
entry and only restore bolted entries on exit, leaving the rest to the
Linux SLB fault handler.

That way we get a really clean way of switching the SLB.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>