cmd/internal/obj/x86: add some more AVX2 instructions

cmd/internal/obj/x86: add some more AVX2 instructions

This adds the VFMADD[213|231]SD, VFNMADD[213|231]SD,
VADDSD, VSUBSD instructions

This will allow us to write a fast path for exp_amd64.s where
these optimizations can be applied in a lot of places.

Change-Id: Ide292107ab887bd1e225a1ad60880235b5ed7c61
Reviewed-on: https://go-review.googlesource.com/61810
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>