selftests/nolibc: add testcases for startup code

The startup code is critical to get the right argc, argv, envp/environ
and _auxv, let's add a startup test group and the corresponding
testcases.

The "environ" test case is also moved from the stdlib test group to this
new startup test group and it is renamed to "environ_envp".

Since argv0 has been used by many other test cases, let's add testcases
to gurantee it too.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>