selftests/x86: Add a test for vDSO unwinding

While the kernel itself doesn't use DWARF unwinding, user code
expects to be able to unwind the vDSO.  The vsyscall
(AT_SYSINFO) entry is manually CFI-annotated, and this tests
that it unwinds correctly.

I tested the test by incorrectly annotating __kernel_vsyscall,
and the test indeed fails if I do that.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/8bf736d1925cdd165c0f980156a4248e55af47a1.1444091584.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>