x86/ibt: Optimize the FineIBT instruction sequence

x86/ibt: Optimize the FineIBT instruction sequence

Scott notes that non-taken branches are faster. Abuse overlapping code
that traps instead of explicit UD2 instructions.

And LEA does not modify flags and will have less dependencies.

Suggested-by: Scott Constable <scott.d.constable@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250224124200.371942555@infradead.org