entry: Provide generic syscall exit function

Like syscall entry all architectures have similar and pointlessly different
code to handle pending work before returning from a syscall to user space.

  1) One-time syscall exit work:
      - rseq syscall exit
      - audit
      - syscall tracing
      - tracehook (single stepping)

  2) Preparatory work
      - Exit to user mode loop (common TIF handling).
      - Architecture specific one time work arch_exit_to_user_mode_prepare()
      - Address limit and lockdep checks
     
  3) Final transition (lockdep, tracing, context tracking, RCU). Invokes
     arch_exit_to_user_mode() to handle e.g. speculation mitigations

Provide a generic version based on the x86 code which has all the RCU and
instrumentation protections right.

Provide a variant for interrupt return to user mode as well which shares
the above #2 and #3 work items.

After syscall_exit_to_user_mode() and irqentry_exit_to_user_mode() the
architecture code just has to return to user space. The code after
returning from these functions must not be instrumented.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200722220519.613977173@linutronix.de