obj/x86: re-organize code to make codegen easier

1. Move AXXX constants (A-enumeration) from "a.out.go" to "aenum.go"
2. Move VEX-encoded optabs from "asm6.go" to "vex_optabs.go"

Also run "go generate" over aenum.go. This explains diff in "anames.go".

Initialization of opindex is split into 2 loops:
one for `vexOptab`, second for `optab`.

Rationale:
when VEX instructions are generated with current structure,
asm6.go is modified, which can lead to merge conflicts and
larger diffs than desired. Same for a.out.go.

This change makes x86avxgen usage possible:
https://go-review.googlesource.com/c/arch/+/66972

Change-Id: Id9eefcf5ccf0a89440e5d01bcb80926a8163b41d
Reviewed-on: https://go-review.googlesource.com/70630
Run-TryBot: Iskander Sharipov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

Author: isharipo