[dev.simd] simd: add carryless multiply

[dev.simd] simd: add carryless multiply

now with comments, and also a test.

choice of data types, method names, etc, are all up for comment.
It's NOT commutative, because of the immediate operand (unless we
swap the bits of the immediate).

Change-Id: I730a6938c6803d0b93544445db65eadc51783e42
Reviewed-on: https://go-review.googlesource.com/c/go/+/726963
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>