start.S: setup stack, SCTRL, and bss

Setup SCTRL register that initializes the cpu with following proporties:
  * 15: RAZ/SBZP disabled
  * 13: V, Normal exception vectors, base address 0x0
  * 12: I, Instruction cache disabled
  * 2:  C, Data and unified caches disabled
  * 0:  M, MMU disabled

Setup stack pointer for each arm modes, and fill BSS section with zeros.