selftests/nolibc: add tests for multi-object linkage

While uncommon, nolibc executables can be linked together from multiple
compilation units.
Add some tests to make sure everything works in that case.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/lkml/20231012-nolibc-linkage-test-v1-1-315e682768b4@weissschuh.net/
Acked-by: Willy Tarreau <w@1wt.eu>