x86/hvm: CFI hardening for hvm_funcs

x86/hvm: CFI hardening for hvm_funcs

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.

In svm.c, make a few rearrangements.  svm_update_guest_cr() has no external
callers so can become static, but needs moving along with svm_fpu_enter() to
avoid a forward declaration.  Move svm_fpu_leave() too, to match.  Also move
svm_update_guest_efer() to drop its forward declaration.

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