[dev.simd] simd: add amd64 emulation for FP SIMD negation

Change-Id: If49f5771bff0baf99ddc0e6ded588b2d9fa50832
Reviewed-on: https://go-review.googlesource.com/c/go/+/783940
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>