stand: Add blake3 support to boot loader

Add the necessary glue to get blake3 building for the boot loaded as
well as connected to the ZFS system so it is useful.

On some platforms, we create references to blake3_sse2_impl and
blake3_sse41_impl ops structs to utilize SIMD. These aren't present on
x86 (since we dind't ask for them), but are on aarch64 with no
implementation. Since we don't want SIMD in the boot loader, have these
all return 'unsupported' always. This should be fixed upstream to allow
more flexibility in this selection, but for now we use this hack to not
modify the sys/contrib/openzfs with difficult to maintain hacks while
an upstreamable solution is found.

tsoome@ did the implementation bits in sys/cddl/boot, and I did the
Makefile work and the aweful blake3_impl_hack.c.

Co-author: tsoome@freebsd.org
Sponsored by: Netflix
Reviewed by: kevans (earlier version)
Differential Revision: https://reviews.freebsd.org/D35750