Support dynamically sized register sets

[andrew]

Summary

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.

Sponsored by: Arm Ltd

[markj]

Doesn't proc_write_regset() need an update too?

[kib]

Is it possible for SVE size to change at runtime by any means? If not, why not initialize the current hw size on boot?

[andrew]

On Linux there is an equivalent of a sysarch call to set it at runtime. I haven't currently implemented that, but store the SVE size for each thread so it can be added later.

[andrew]

Also support proc_write_regset