x86: Remove inline for lapic access routines

Remove inline for lapic access routines and expose lapic_read()
& lapic_write() as APIs to read/write lapic registers. Also move
stop_this_cpu() to mp_init.c as it has nothing to do with lapic.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>