mac: Enable process-wide CPU mitigations where available.

The existing mitigation flag is per-thread, so this offers stronger
protections. The thread-specific mitigation will remain enabled in
order to support older macOS versions. The new process-wide flag does
not conflict with it (task takes precedence over thread).

The posix_spawnattr option is used instead of the libproc API, because
the latter requires permitting (allow system-sched (target self)) in the
sandbox, which would grant additional capabilities beyond the CPU
mitigations control.

Bug: 1129602
Change-Id: I876f2aae3a3e26a622c67ad181efbdfd91fa8b76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2722913
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858301}