message: Systemtime: Remove static mut
diff: use core::hint::spin_loop;
  -use core::num::NonZeroU64;
  use hermit_entry::boot_info::PlatformInfo;
  -use hermit_sync::without_interrupts;
  +use hermit_sync::{without_interrupts, OnceCell};
  use time::OffsetDateTime;
  use x86::io::*;
  @@ -171,10 +170,10 @@ impl Rtc {
  }
  }
  -static mut BOOT_TIME: Option<NonZeroU64> = None;
  +static BOOT_TIME: OnceCell<u64> = OnceCell::new();
  pub fn get_boot_time() -> u64 {
  - unsafe { BOOT_TIME.unwrap().get() }
  + *BOOT_TIME.get().unwrap()
  }
  pub fn init() {
  @@ -191,7 +190,5 @@ pub fn init() {
  info!("HermitCore-rs booted on {boot_time}");
  let micros = u64::try_from(boot_time.unix_timestamp_nanos() / 1000).unwrap();
  - unsafe {
  - BOOT_TIME = Some(micros.try_into().unwrap());
  - }
  + BOOT_TIME.set(micros).unwrap();
  }
  