armv8crypto: Extract GCM state into a structure

This makes it easier to refactor the GCM code to operate on
crypto_buffer_cursors rather than plain contiguous buffers, with the aim
of minimizing the amount of copying and zeroing done today.

No functional change intended.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Ampere Computing
Submitted by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D28500