Kernel: Implement TimeManagement::boot_time() for aarch64

For now just return 0 as we have no RTC support on aarch64 yet, and add
a FIXME to return the correct value.

Kernel/Time/TimeManagement.cpp

@@ -231,7 +231,8 @@ Time TimeManagement::boot_time()
 #if ARCH(X86_64)
     return RTC::boot_time();
 #elif ARCH(AARCH64)
-    TODO_AARCH64();
+    // FIXME: Return correct boot time
+    return Time::from_seconds(0);
 #else
 #    error Unknown architecture
 #endif