[arch] relocate binary to proper physical location at boot, add arch_chain_load

-in arm start.S, calculate and move the current binary to the proper physical
location before enabling the mmu.
-add arch_chain_load which does the necessary translations from virtual to
physical, tries to gracefully shut the system down, and branches into the loaded binary.

arch/arm64/arch.c

@@ -51,3 +51,9 @@ void arch_idle(void)
     __asm__ volatile("wfi");
 }
 
+void arch_chain_load(void *entry)
+{
+    PANIC_UNIMPLEMENTED;
+}
+
+