arm64: Correctly align the SVE signal context

The SVE signal context needs to be correctly aligned. Fix this by
creating a new macro to calculate the needed size to provide this
alignment, and use it when setting and checking the saved SVE signal
context.

Reported by: cperciva
Reviewed by: cperciva, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54396

(cherry picked from commit a9e77eb7016df70723c208fc09fbd01ec23a732d)