drivers/timer/loapic_timer.c: migrate to new local APIC accessors

More clearly differentiate MVIC vs. APIC timer code, and use new APIC
accessors in include/drivers/loapic.h. Remove extraneous comments, and
other light cleanup work.

This driver is in need of a serious overhaul -- despite appearing to
have support for TICKLESS_KERNEL and DEVICE_POWER_MANAGEMENT, bitrot
has taken its toll and the driver will not build with these enabled.
These should be removed or made to work... but not in this patch.

Old x2APIC-related accessors in kernel_arch_func.h are eliminated.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>
diff --git a/drivers/interrupt_controller/loapic_intr.c b/drivers/interrupt_controller/loapic_intr.c
index 55049d9..a9e325b 100644
--- a/drivers/interrupt_controller/loapic_intr.c
+++ b/drivers/interrupt_controller/loapic_intr.c
@@ -64,6 +64,20 @@
 static u32_t loapic_device_power_state = DEVICE_PM_ACTIVE_STATE;
 #endif
 
+/*
+ * this should not be a function at all, really, it should be
+ * hand-coded in include/drivers/sysapic.h. but for now it remains
+ * a function, just moved here from drivers/timer/loapic_timer.c
+ * where it REALLY didn't belong.
+ */
+
+#ifdef CONFIG_X2APIC
+void z_x2apic_eoi(void)
+{
+	x86_write_x2apic(LOAPIC_EOI, 0);
+}
+#endif
+
 /**
  *
  * @brief Initialize the Local APIC or xAPIC