core: Add compiler barrier semantic to cpu_relax

Will eventually help to get rid of volatile for several synchronization
variables by enforcing a re-read in busy-wait loops:

        while (state_var == STATE)
                cpu_relax();

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Fix some minor style issues

Found by checkpatch. No functional changes.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Introduce per-page TLB flushing

Reduce the overhead of MMIO parsing specifically by introducing a
per-page TLB flush. Restrict the existing global one to x86, that's
where is is used so far.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>