selftests/nolibc: only run constructor tests on nolibc

selftests/nolibc: only run constructor tests on nolibc

The nolibc testsuite can be run against other libcs to test for
interoperability. Some aspects of the constructor execution are not
standardized and musl does not provide all tested feature, for one it
does not provide arguments to the constructors, anymore?

Skip the constructor tests on non-nolibc configurations.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250212-nolibc-test-constructor-v1-1-c963875b3da4@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>