Migrate to the 3.14 longterm branch

* Include generic asm/preempt.h header.
	"sched, arch: Create asm/preempt.h"
		a787870924dbd6f321661e06d4ec1c7a408c9ccf
* Include generic asm/hash.h header.
	"lib: introduce arch optimized hash library"
		71ae8aac3e198c6f3577cb7ad3a17f6128e97bfa
Support catching of signals by user-mode handlers

* Extend the signal delivery implementation for system call restarting,
  creation of signal stack frames, invocation of arbitrary handlers installed
  with sigaction(2), and restoration of the execution context after termination.
* Introduce a vDSO (virtual dynamic shared object) to provide a trampoline for
  sys_rt_sigreturn, so as to avoid requiring an executable stack.
* Remove some unused cruft from the asm/elf.h header.
* Fix handle_exception_tail to use the saved value of the status CSR in struct
  pt_regs instead of its current value.  Since nested execution of kernel
  control paths would cause the PS bit to be set unexpectedly, the CSR's
  present value cannot be relied upon to discern whether or not the exception
  handler would be returning to userspace.
  Problematic behavior was initially observed in the context of an interrupted
  sys_nanosleep call, wherein the bug would withhold resume_userspace from
  executing and thus prevent a proper restart of the system call through the
  ERESTART_RESTARTBLOCK mechanism.