um: Split out default elf_aux_hwcap

um: Split out default elf_aux_hwcap

Setting all auxiliary vector values to default values if one of them
was not provided by the host will discard perfectly fine values.

Remove the elf_aux_platform fallback from the vDSO ones.
As zero is the correct fallback anyways, don't create a new conditional.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20251028-uml-remove-32bit-pseudo-vdso-v1-3-e930063eff5f@weissschuh.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>