selftests: Add kselftest for syscall user dispatch

Implement functionality tests for syscall user dispatch.  In order to
make the test portable, refrain from open coding syscall dispatchers and
calculating glibc memory ranges.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201127193238.821364-6-krisman@collabora.com