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 <[e-mail]>
Signed-off-by: Stafford Horne <[e-mail]>

arch/openrisc/Kconfig

···
 	select HAVE_UID16
 	select HAVE_PAGE_SIZE_8KB
 	select HAVE_REGS_AND_STACK_ACCESS_API
+	select HAVE_RSEQ
 	select GENERIC_ATOMIC64
 	select GENERIC_CLOCKEVENTS_BROADCAST
 	select GENERIC_SMP_IDLE_THREAD

arch/openrisc/kernel/entry.S

···
  * interrupts that set NEED_RESCHED or SIGNALPENDING... really true? */
 
 _syscall_check_work:
+#ifdef CONFIG_DEBUG_RSEQ
+	l.jal	rseq_syscall
+	 l.ori	r3,r1,0
+#endif
 	/* Here we need to disable interrupts */
 	DISABLE_INTERRUPTS(r27,r29)
 	TRACE_IRQS_OFF

arch/openrisc/kernel/signal.c

···
 {
 	int ret;
 
+	rseq_signal_deliver(ksig, regs);
+
 	ret = setup_rt_frame(ksig, sigmask_to_save(), regs);
 
 	signal_setup_done(ret, ksig, test_thread_flag(TIF_SINGLESTEP));