core-entry-2020-12-14

 - More generalization of entry/exit functionality

 - The consolidation work to reclaim TIF flags on x86 and also for non-x86
   specific TIF flags which are solely relevant for syscall related work
   and have been moved into their own storage space. The x86 specific part
   had to be merged in to avoid a major conflict.

 - The TIF_NOTIFY_SIGNAL work which replaces the inefficient signal
   delivery mode of task work and results in an impressive performance
   improvement for io_uring. The non-x86 consolidation of this is going to
   come seperate via Jens.

 - The selective syscall redirection facility which provides a clean and
   efficient way to support the non-Linux syscalls of WINE by catching them
   at syscall entry and redirecting them to the user space emulation. This
   can be utilized for other purposes as well and has been designed
   carefully to avoid overhead for the regular fastpath. This includes the
   core changes and the x86 support code.

 - Simplification of the context tracking entry/exit handling for the users
   of the generic entry code which guarantee the proper ordering and
   protection.

 - Preparatory changes to make the generic entry code accomodate S390
   specific requirements which are mostly related to their syscall restart
   mechanism.