powerpc/pseries: Fix scv instruction crash with kexec

In the Linux kernel, the following vulnerability has been resolved:

powerpc/pseries: Fix scv instruction crash with kexec

kexec on pseries disables AIL (reloc_on_exc), required for scv

instruction support, before other CPUs have been shut down. This means

they can execute scv instructions after AIL is disabled, which causes an

interrupt at an unexpected entry location that crashes the kernel.

Change the kexec sequence to disable AIL after other CPUs have been

brought down.

As a refresher, the real-mode scv interrupt vector is 0x17000, and the

fixed-location head code probably couldn't easily deal with implementing

such high addresses so it was just decided not to support that interrupt

at all.