selftests: vDSO: chacha: Correctly skip test if necessary

According to kselftest.h ksft_exit_skip() is not meant to be called when
a plan has already been printed.

Use the recommended function ksft_test_result_skip().

This fixes a bug, where the TAP output would be invalid when skipping:

	TAP version 13
	1..1
	ok 2 # SKIP Not implemented on architecture

The SKIP line should start with "ok 1" as the plan only contains one test.

Fixes: 3b5992ea ("selftests: vDSO: unconditionally build chacha test")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/all/20250611-selftests-vdso-fixes-v3-1-e62e37a6bcf5@linutronix.de