openrisc: Add support for restartable sequences

Implement support for restartable sequences on OpenRISC by doing:
 - Select HAVE_RSEQ in Kconfig
 - Call rseq_syscall() on return to userspace when CONFIG_DEBUG_RSEQ
   is enabled.
 - Call rseq_signal_deliver() to fixup the pre-signal stack frame when a
   signal is delivered on top of a restartable sequence critical section

Cc: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>