math/big: new mini-compiler for arith assembly

The arith assembly is big enough, and the details that you have to keep
in mind are complex enough and varied enough, that it is worth using
a Go program to generate the assembly. That way, all the architectures
can use the same algorithms, and porting to new architectures will be
easier.

This is the first of a sequence of CLs to introduce a new mini-compiler
for generating the arith assembly, in math/big/internal/asmgen.
This CL has the basics of the compiler as well as a couple simple
architectures and the generator for addVV/subVV. It does not check
in the generated assembly yet. That will happen in a followup CL after
the other architectures and generators have been added.

Change-Id: Ib704c60fd972fc5690ac04d8fae3712ee2c1a80a
Reviewed-on: https://go-review.googlesource.com/c/go/+/664935


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>