selftests: vDSO: parse_vdso: Drop vdso_init_from_auxv()

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This also removes the usage of ElfXX_auxv_t, which is not formally
standardized.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/all/20250226-parse_vdso-nolibc-v2-11-28e14e031ed8@linutronix.de