x86: Introduce alternative indirect thunks

x86: Introduce alternative indirect thunks

Depending on hardware and microcode availability, we will want to replace
IND_THUNK_REPOLINE with other implementations.

For AMD hardware, choose IND_THUNK_LFENCE in preference to retpoline if lfence
is known to be (or was successfully made) dispatch serialising.

This is part of XSA-254.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 858cba0d4c6b6b45180afcb41561fd6585ad51a3
master date: 2018-01-16 17:45:50 +0000