x86/bugframe: CFI hardening

x86/bugframe: 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.

run_in_exception_handler() managed to escape typechecking, as the compiler
can't see where function pointer gets called.  After adding some ad-hoc
typechecking, it turns out that dump_execution_state() alone differs in
const-ness from the other users of run_in_exception_handler().

Introduce a new show_execution_state_nonconst() to make the typechecking
happy.

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