arch: arc: add user space support for arc

* add the implementation of syscall
  * based on 'trap_s' intruction, id = 3
* add the privilege stack
  * the privilege stack is allocted with thread stack
  * for the kernel thread, the privilege stack is also a
    part of thread stack, the start of stack can be configured
    as stack guard
  * for the user thread, no stack guard, when the user stack is
    overflow, it will fall into kernel memory area which requires
    kernel privilege, privilege violation will be raised
* modify the linker template and add MPU_ADDR_ALIGN
* add user space corresponding codes in mpu
* the user sp aux reg will be part of thread context
* When user thread is interruptted for the 1st time, the context is
  saved in user stack (U bit of IRQ_CTLR is set to 1). When nest
  interrupt comes, the context is saved in thread's privilege stack
* the arc_mpu_regions.c is moved to board folder, as it's board
  specific
* the above codes have been tested through tests/kernel/mem_protect/
  userspace for MPU version 2

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>