um: properly align signal stack on x86_64

The stack needs to be properly aligned so 16 byte memory accesses on the
stack are correct. This was broken when introducing the dynamic math
register sizing as the rounding was not moved appropriately.

Fixes: 3f17fed2 ("um: switch to regset API and depend on XSTATE")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250107133509.265576-1-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>