x86/alternatives: Make FineIBT mode Kconfig selectable

x86/alternatives: Make FineIBT mode Kconfig selectable

Since FineIBT performs checking at the destination, it is weaker against
attacks that can construct arbitrary executable memory contents. As such,
some system builders want to run with FineIBT disabled by default. Allow
the "cfi=kcfi" boot param mode to be selectable through Kconfig via the
newly introduced CONFIG_CFI_AUTO_DEFAULT.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240501000218.work.998-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>