x86/cfi: Boot time selection of CFI scheme

Add the "cfi=" boot parameter to allow people to select a CFI scheme
at boot time. Mostly useful for development / debugging.

Requested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221027092842.699804264@infradead.org