x86/Kconfig: Enable kernel IBT by default

The kernel IBT defense strongly mitigates the common "first step" of ROP
attacks, by eliminating arbitrary stack pivots (that appear either at
the end of a function or in immediate values), which cannot be reached
if indirect calls must be to marked function entry addresses. IBT is
also required to be enabled to gain the FineIBT feature when built with
Kernel Control Flow Integrity.

Additionally, given that this feature is runtime enabled via CPU ID,
it clearly should be built in by default; it will only be enabled if the
CPU supports it. The build takes 2 seconds longer, which seems a small
price to pay for gaining this coverage by default.

Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221101172503.gonna.094-kees@kernel.org