Kernel::pause_current_thread can be implemented much simpler and is not
restricted to core threads, in contrast to Kernel::pause_thread which
also benefits from the split and can be moved to core_interface.h.
ref #1101
When using the initial SP of a main thread for the UTCB
startup-argument, fork_trampoline in libc_noux gets broken.
The function expects the SP to be initialized already in contrast
to the _start function in crt0.s that is called for processes that
are not forked. As the main-thread UTCB is located at the same virtual
address for every PD anyways, we can circumvent this problem by
defining it statically.
ref #964
