x86/early_printk: Harden early_serial

Scott found that mem32_serial_in() is an ideal speculation gadget, an
indirectly callable function that takes an adddress and offset and
immediately does a load.

Use static_call() to take away the need for indirect calls and
explicitly seal the functions to ensure they're not callable on IBT
enabled parts.

Reported-by: Scott Constable <scott.d.constable@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>