drivers: loapic: move 'z_loapic_int_vec_set()' into pinned section

Move it out of boot section because it's also called by none-boot function
'loapic_resume()' at runtime. Better to keep boot-only things in boot
section to avoid paging in boot section things at runtime.

Signed-off-by: Dong Wang <dong.d.wang@intel.com>