selftests: vDSO: vdso_test_abi: Use explicit indices for name array

The array relies on the numeric values of the clock IDs.
When reading the code it is not obvious that the order is correct.

Make the code easier to read by using explicit indices.

While at it make the array static.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250812-vdso-tests-fixes-v2-5-90f499dd35f8@linutronix.de