x86/speculation: Use generic retpoline by default on AMD

x86/speculation: Use generic retpoline by default on AMD

commit 244d00b5dd4755f8df892c86cab35fb2cfd4f14b upstream.

AMD retpoline may be susceptible to speculation. The speculation
execution window for an incorrect indirect branch prediction using
LFENCE/JMP sequence may potentially be large enough to allow
exploitation using Spectre V2.

By default, don't use retpoline,lfence on AMD.  Instead, use the
generic retpoline.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>