selftests/nolibc: implement a few tests for various syscalls

This adds 63 tests covering about 34 syscalls. Both successes and
failures are tested. Two tests fail when run as unprivileged user
(link_dir which returns EACCESS instead of EPERM, and chroot which
returns EPERM). One test (execve("/")) expects to fail on EACCESS,
but needs to have valid arguments otherwise the kernel will log a
message. And a few tests require /proc to be mounted.

The code is not pretty since all tests are one-liners, sometimes
resulting in long lines, especially when using compount statements to
preset a line, but it's convenient and doesn't obfuscate the code,
which is important to understand what failed.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>