x86/spec-ctrl: Extend repoline safey calcuations for eIBRS and Atom parts

x86/spec-ctrl: Extend repoline safey calcuations for eIBRS and Atom parts

All currently-released Atom processors are in practice retpoline-safe, because
they don't fall back to a BTB prediction on RSB underflow.

However, an additional meaning of Enhanced IRBS is that the processor may not
be retpoline-safe.  The Gemini Lake platform, based on the Goldmont Plus
microarchitecture is the first Atom processor to support eIBRS.

Until Xen gets full eIBRS support, Gemini Lake will still be safe using
regular IBRS.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 17f74242ccf0ce6e51c03a5860947865c0ef0dc2
master date: 2019-03-18 16:26:40 +0000