aa64: finalise context switch, page fault handler and syscall

aa64: finalise context switch, page fault handler and syscall

* context switch: pre-switch signposting check, signal handling and ttbr
  switching
* mm_abort: complement page fault handler, fault context gathering for
  aa64
* klibc: aa64 optimization of str/mem functions
* page:  page permission fix; proper use of AF and AP[2]
* irq:   async exception handling with gic framework
* other fix arounds and refactorings