blake3: fix up bogus checksums in face of cpu migration

This is a temporary measure until a better fix is sorted out.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Sponsored by: Rubicon Communications, LLC ("Netgate")
Closes #14785
Closes #14808