proc: provide details on indirect branch speculation

Similar to speculation store bypass, show information about the indirect
branch speculation mode of a task in /proc/$pid/status.

For testing/benchmarking, I needed to see whether IB (Indirect Branch)
speculation (see Spectre-v2) is enabled on a task, to see whether an
IBPB instruction should be executed on an address space switch.
Unfortunately, this information isn't available anywhere else and
currently the only way to get it is to hack the kernel to expose it
(like this change).  It also helped expose a bug with conditional IB
speculation on certain CPUs.

Another place this could be useful is to audit the system when using
sanboxing.  With this change, I can confirm that seccomp-enabled
process have IB speculation force disabled as expected when the kernel
command line parameter `spectre_v2_user=seccomp`.

Since there's already a 'Speculation_Store_Bypass' field, I used that
as precedent for adding this one.

[amistry@google.com: remove underscores from field name to workaround documentation issue]
  Link: https://lkml.kernel.org/r/20201106131015.v2.1.I7782b0cedb705384a634cfd8898eb7523562da99@changeid

Link: https://lkml.kernel.org/r/20201030172731.1.I7782b0cedb705384a634cfd8898eb7523562da99@changeid
Signed-off-by: Anand K Mistry <amistry@google.com>
Cc: Anthony Steinhauser <asteinhauser@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Anand K Mistry <amistry@google.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Alexey Gladkov <gladkov.alexey@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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