selftests/x86: Do not rely on "int $0x80" in test_mremap_vdso.c

On 64-bit builds, we should not rely on "int $0x80" working (it only does if
CONFIG_IA32_EMULATION=y is enabled).

Without this patch, the move test may succeed, but the "int $0x80" causes
a segfault, resulting in a false negative output of this self-test.

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kselftest@vger.kernel.org
Cc: shuah@kernel.org
Link: http://lkml.kernel.org/r/20180211111013.16888-4-linux@dominikbrodowski.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/x86/test_mremap_vdso.c

@@ -90,8 +90,12 @@ int main(int argc, char **argv, char **envp)
 			vdso_size += PAGE_SIZE;
 		}
 
+#ifdef __i386__
 		/* Glibc is likely to explode now - exit with raw syscall */
 		asm volatile ("int $0x80" : : "a" (__NR_exit), "b" (!!ret));
+#else /* __x86_64__ */
+		syscall(SYS_exit, ret);
+#endif
 	} else {
 		int status;