cmd/internal/obj/x86: add AVX2 gather and VSIB

cmd/internal/obj/x86: add AVX2 gather and VSIB

Enables AVX2 gather instructions and VSIB support,
which makes vm32{x,y} vm64{x,y} operands encodable.

AXXX constants placed with respect to sorting order.
New VEX optabs inserted near non-VEX entries to simplify
potential transition to auto-generated VSIB optabs.

Tests go into new AMD64 encoder test file (amd64enc_extra.s)
to avoid unnecessary interactions with auto-generated "amd64enc.s".

Side note: x86avxgen did not produced these instructions
because x86.v0.2.csv misses them.
This also explains why x86 test suite have no AVX2 gather
instructions tests.

List of new instructions:
  VGATHERPDP
  VGATHERDPS
  VGATHERQPD
  VGATHERQPS
  VPGATHERDD
  VPGATHERDQ
  VPGATHERQD
  VPGATHERQQ

Change-Id: Iac852f3c5016523670bd99de6bec6a48f66fb4f6
Reviewed-on: https://go-review.googlesource.com/77970
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>