Introduce the scheduler's 'sched_smt_present' static key and provide
the query function 'sched_smt_active()' to be used in subsequent x86
speculation code.

Loosely based on the following upstream commits:
  - 321a874a ("sched/smt: Expose sched_smt_present static key")
  - c5511d03 ("sched/smt: Make sched_smt_present track topology")
  - ba2591a5 ("sched/smt: Update sched_smt_present at runtime")
  - 1b568f0a ("sched/core: Optimize SCHED_SMT")

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>