x86/stack: CFI hardening

x86/stack: CFI hardening

Control Flow Integrity schemes use toolchain and optionally hardware support
to help protect against call/jump/return oriented programming attacks.

Use cf_check to annotate function pointer targets for the toolchain.

The function typecheck in switch_stack_and_jump() is incompatible with control
flow typechecking.  It's ok for reset_stack_and_jump_ind(), but for
reset_stack_and_jump(), it would force us to endbr64 the targets which are
branched to directly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>