Support dynamically sized register sets

We don't always know the size of the register set at compile time,
e.g. on arm64 the size of the SVE registers need to be queried on boot.
To support register sets that needs to be calculated at run time
query the correct size when it is zero.

Reviewed by: markj, kib (earlier version)
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41302