Import Blake2 algorithms (blake2b, blake2s) from libb2

The upstream repository is on github BLAKE2/libb2. Files landed in
sys/contrib/libb2 are the unmodified upstream files, except for one
difference: secure_zero_memory's contents have been replaced with
explicit_bzero() only because the previous implementation broke powerpc
link. Preferential use of explicit_bzero() is in progress upstream, so
it is anticipated we will be able to drop this diff in the future.

sys/crypto/blake2 contains the source files needed to port libb2 to our
build system, a wrapped (limited) variant of the algorithm to match the API
of our auth_transform softcrypto abstraction, incorporation into the Open
Crypto Framework (OCF) cryptosoft(4) driver, as well as an x86 SSE/AVX
accelerated OCF driver, blake2(4).

Optimized variants of blake2 are compiled for a number of x86 machines
(anything from SSE2 to AVX + XOP). On those machines, FPU context will need
to be explicitly saved before using blake2(4)-provided algorithms directly.
Use via cryptodev / OCF saves FPU state automatically, and use via the
auth_transform softcrypto abstraction does not use FPU.

The intent of the OCF driver is mostly to enable testing in userspace via
/dev/crypto. ATF tests are added with published KAT test vectors to
validate correctness.

Reviewed by: jhb, markj
Obtained from: github BLAKE2/libb2
Differential Revision: https://reviews.freebsd.org/D14662