crypto/internal/fips140/aes: actually use the VTBL instruction on arm64

Support for the VTBL instruction was added in CL 110015 - use it
directly, rather than using WORD encodings. Note that one of the
WORD encodings does not actually match the instruction in the
comment - use the instruction that matches the existing encoding
instead.

Change-Id: I1933162f8144a6b86b38e8b550d36907131b1dd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/666795


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>