x86, selftests: Add sigreturn selftest

x86, selftests: Add sigreturn selftest

This is my sigreturn test, added mostly unchanged from its old
home. It exercises the sigreturn(2) syscall, specifically
focusing on its interactions with various IRET corner cases.

It tests for correct behavior in several areas that were
historically dangerously buggy. For example, it exercises espfix
on kernels of both bitnesses under various conditions, and it
contains testcases for several now-fixed bugs in IRET error
handling.

If you run it on older kernels without the fixes, your system will
crash. It probably won't eat your data in the process.

There is no released kernel on which the sigreturn_64 test will
pass, but it passes on tip:x86/asm.

I plan to switch to lib.mk for Linux 4.2.

I'm not using the ksft_ helpers at all yet.  I can do that later.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
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: Denys Vlasenko <vda.linux@googlemail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Shuah Khan <shuah.kh@samsung.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/89d10b76b92c7202d8123654dc8d36701c017b3d.1428386971.git.luto@kernel.org
[ Fixed empty format string GCC build warning in trivial_32bit_program.c ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>