simd/archsimd: add HasAVX2() guards to tests that need them

simd/archsimd: add HasAVX2() guards to tests that need them

This may not be complete, because some 256-bit float operations
may be implemented with integer simd instead (in some cases there
is no float op).  (And some tests may have just been overlooked.)

Fixes #76866.

Change-Id: I90b1dfe039c322c484af916436fc6f6c50a31030
Reviewed-on: https://go-review.googlesource.com/c/go/+/731502
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>