locking/barriers: introduce new memory barrier gmb()

locking/barriers: introduce new memory barrier gmb()

CVE-2017-5753
CVE-2017-5715

In constrast to existing mb() and rmb() barriers,
gmb() barrier is arch-independent and can be used to
implement any type of memory barrier.
In x86 case, it is either lfence or mfence, based on
processor type. ARM and others can define it according
to their needs.

Suggested-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>