Revert "csky: Add support for restartable sequence"

This reverts commit 9866d141a0.

The csky rseq support has been merged without ever notifying the rseq
maintainers, and without any of the required asssembler glue in the rseq
selftests, which means it is entirely untested.

It is also derived from a non-upstream riscv patch which has known bugs.

The assembly part of this revert should be carefully reviewed by the
architecture maintainer because it touches code which has changed since
the merge of the reverted patch.

The rseq selftests assembly glue should be introduced at the same time
as the architecture rseq support. Without the presence of any test, I
recommend reverting rseq support from csky for now.

Link: https://lore.kernel.org/lkml/1257037909.25426.1626705790861.JavaMail.zimbra@efficios.com/
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-csky@vger.kernel.org

arch/csky/Kconfig

@@ -94,7 +94,6 @@ config CSKY
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_REGS_AND_STACK_ACCESS_API
-	select HAVE_RSEQ
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select MAY_HAVE_SPARSE_IRQ

arch/csky/kernel/entry.S

@@ -50,10 +50,6 @@ ENTRY(csky_systemcall)
 	SAVE_ALL TRAP0_SIZE
 	zero_fp
 	context_tracking
-#ifdef CONFIG_RSEQ_DEBUG
-	mov     a0, sp
-	jbsr    rseq_syscall
-#endif
 	psrset  ee, ie
 
 	lrw     r9, __NR_syscalls

arch/csky/kernel/signal.c

@@ -179,8 +179,6 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 	sigset_t *oldset = sigmask_to_save();
 	int ret;
 
-	rseq_signal_deliver(ksig, regs);
-
 	/* Are we from a system call? */
 	if (in_syscall(regs)) {
 		/* Avoid additional syscall restarting via ret_from_exception */