locking/barriers: introduce new observable speculation barrier

locking/barriers: introduce new observable speculation barrier

CVE-2017-5753 (Spectre v1 Intel)

The new observable speculation barrier, osb(), ensures
that any user observable speculation doesn't cross the boundary.

Any user observable speculative activity on this CPU
thread before this point either completes, reaches a
state it can no longer cause an observable activity, or
is aborted before instructions after the barrier execute.

In x86 case, osb() resolves in lfence if X86_FEATURE_LFENCE_RDTSC
is present. Other architectures can define their variants.

Suggested-by: Arjan van de Ven <arjan@linux.intel.com>
Suggested-by: Alan Cox <alan.cox@intel.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>