cmd/internal/obj/wasm: refactor handling of wasm variables

cmd/internal/obj/wasm: refactor handling of wasm variables

This commit improves how registers get mapped to wasm variables. This
is a preparation for future improvements (e.g. adding 32 bit float
registers).

Change-Id: I374c80b2d6c9bcce6b0e373fe921b5ad4dee40ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/191777
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>