x86, selftests: Add single_step_syscall test

x86, selftests: Add single_step_syscall test

This is a very simple test that makes system calls with TF set.

This test currently fails when running the 32-bit build on a
64-bit kernel on an Intel CPU.  This bug will be fixed by the
next commit.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Shuah Khan <shuah.kh@samsung.com>
Link: http://lkml.kernel.org/r/20e68021155f6ab5c60590dcad81d37c68ea2c4f.1429139075.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>