Use ITC on IA64 rather than broken routine based on daytime.

* grub-core/kern/efi/efi.c (grub_rtc_get_time_ms) [__ia64__]: Remove on
	ia64.
	(grub_get_rtc) [__ia64__]: Likewise.
	* grub-core/kern/ia64/efi/init.c (divisor): New variable.
	(get_itc): New function.
	(grub_rtc_get_time_ms): Likewise.
	(grub_machine_init): Calibrate ITC.
	* include/grub/efi/time.h (grub_get_rtc), (GRUB_TICKS_PER_SECOND):
	Keep only on non-ia64. Don't export since it's broken and used only
	if TSC is unavailable.

grub-core/kern/efi/efi.c

@@ -230,6 +230,7 @@ grub_efi_get_variable (const char *var, const grub_efi_guid_t *guid,
   return NULL;
 }
 
+#ifndef __ia64__
 grub_uint64_t
 grub_rtc_get_time_ms (void)
 {
@@ -250,6 +251,7 @@ grub_get_rtc (void)
 {
   return grub_rtc_get_time_ms ();
 }
+#endif
 
 #pragma GCC diagnostic ignored "-Wcast-align"