Add support for hppa/PA-RISC architecture

Add the missing architecture specific functions to provide support for
the hppa/PA-RISC architecture:
- the processor internal time stamp counter (Control Register CR16) is
  used to get high-performance/low-latency cycle counts
- gcc provides the necessary built-in atomic functions on hppa (which in
  turn uses the light-weigth atomic locking syscall-interface of the
  Linux kernel)

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>