nova: extent ec_ctrl in kernel and base-nova

The commit switches the kernel branch to r5 and updates the syscall bindings
in base-nova accordingly. Beside some cleanups, r5 extents the ec_ctrl syscall
to support following features:

* An EC may yield its current timeslice. The timeslice gets enqueued at the end
  of the run queue without refreshing the left budget of the quantum.
* An EC helps with the current SC another EC. Used in Genode to implement
  helping over a user-level lock to avoid live-lock situation. (Think of
  a limited priority-inheritance mechanism.)
* An EC requests a new scheduler decision. Used in Genode to transfer back the
  potentially helping SC to the helper EC.

Issue #986
nova: set pager and native ec cap for main thread

Prerequisite for follow up commit regarding base-nova specific Genode Spinlock
replacement.

Issue #986
base: support cmpxchg with different sizes

Issue #986
nova: replacement of generic Genode spinlock

Turn Genode user-level spinlock into a user-level "helpinglock". This requires
support by the kernel introduced with kernel branch r5.

The commit avoids live-locks when multiple threads with SCs on different
priority levels compete for the same user-level Genode "spinlock".

Issue #986
nova: support usage of priorities in Genode/Nova

Enables finally the usage of priorities on base-nova.

Fixes #986