Support interrupt stuffing for Skylake

[Andi Kleen]

Skylake with retpoline needs additional protection against return
stack underflows. While full coverage is more involved, there
are some low hanging fruits that are relatively easy.  We already
did stuffing on context switch earlier, this adds the next
low hanging fruit: stuffing on interrupts and exception return
to the kernel. 

We only need to stuff when returning to the kernel so there is
no impact on anything interrupting user space directly. Also the
additional stuffing is only enabled on Skylake.

Git tree available here
git://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git spec/interrupt-stuff-1

v1: Initial post, but based on the earlier skl-rsb code.