x86/selftests: Add clobbers for int80 on x86_64

Kernel erases R8..R11 registers prior returning to userspace
from int80:

  https://lkml.org/lkml/2009/10/1/164

GCC can reuse these registers and doesn't expect them to change
during syscall invocation. I met this kind of bug in CRIU once
GCC 6.1 and CLANG stored local variables in those registers
and the kernel zerofied them during syscall:

  990d33f1a1

By that reason I suggest to add those registers to clobbers
in selftests.  Also, as noted by Andy - removed unneeded clobber
for flags in INT $0x80 inline asm.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: 0x7f454c46@gmail.com
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kselftest@vger.kernel.org
Link: http://lkml.kernel.org/r/20170213101336.20486-1-dsafonov@virtuozzo.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/x86/ptrace_syscall.c

@@ -58,7 +58,8 @@ static void do_full_int80(struct syscall_args32 *args)
 	asm volatile ("int $0x80"
 		      : "+a" (args->nr),
 			"+b" (args->arg0), "+c" (args->arg1), "+d" (args->arg2),
-			"+S" (args->arg3), "+D" (args->arg4), "+r" (bp));
+			"+S" (args->arg3), "+D" (args->arg4), "+r" (bp)
+			: : "r8", "r9", "r10", "r11");
 	args->arg5 = bp;
 #else
 	sys32_helper(args, int80_and_ret);