arm64: uti: Add arch-dependent helper for context switch

arm64 performs context-switch in kernel space instead of user space as in
x86_64.

Change-Id: Ib119b9ff014effb970183ee86cfac67fab773cba
Futjitsu: POSTK_DEBUG_ARCH_DEP_99

executer/user/arch/x86_64/archdep.S

@@ -17,6 +17,8 @@ Syscam call convention:
   rdi:	 fd
   rsi:	cmd
   rdx:	param
+
+  Save and switch the context including FS.
 */
 
 .global switch_ctx