alpha: fix a 14 years old bug in sigreturn tracing

The way sigreturn() is implemented on alpha breaks PTRACE_SYSCALL,
all way back to 1.3.95 when alpha has grown PTRACE_SYSCALL support.

What happens is direct return to ret_from_syscall, in order to bypass
mangling of a3 (error indicator) and prevent other mutilations of
registers (e.g. by syscall restart).  That's fine, but... the entire
TIF_SYSCALL_TRACE codepath is kept separate on alpha and post-syscall
stopping/notifying the tracer is after the syscall.  And the normal
path we are forcibly switching to doesn't have it.

So we end up with *one* stop in traced sigreturn() vs. two in other
syscalls.  And yes, strace is visibly broken by that; try to strace
the following
	#include <signal.h>
	#include <stdio.h>
	void f(int sig) {}
	main()
	{
		signal(SIGHUP, f);
		raise(SIGHUP);
		write(1, "eeeek\n", 6);
	}
and watch the show.  The
	close(1)                                = 405
in the end of strace output is coming from return value of write() (6 ==
__NR_close on alpha) and syscall number of exit_group() (__NR_exit_group ==
405 there).

The fix is fairly simple - the only thing we end up missing is the call
of syscall_trace() and we can tell whether we'd been called from the
SYSCALL_TRACE path by checking ra value.  Since we are setting the
switch_stack up (that's what sys_sigreturn() does), we have the right
environment for calling syscall_trace() - just before we call
undo_switch_stack() and return.  Since undo_switch_stack() will overwrite
s0 anyway, we can use it to store the result of "has it been called from
SYSCALL_TRACE path?" check.  The same thing applies in rt_sigreturn().

Tested-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>