x86/debug: Move historical SYSENTER junk into exc_debug_kernel()

The historical SYSENTER junk is explicitly for from-kernel, so move it
to the #DB-from-kernel handler.

It is ordered after the notifier, which is important for KGDB which uses TF
single-step and needs to consume the event before that point.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.031099736@infradead.org