cmd/link/internal/wasm: optimize data section in wasm binary

This change optimizes the data section in the wasm binary by
omitting blocks of zeroes and instead emitting data segments
with offsets skipping the zeroes.

This optimization is inspired by the memory-packing pass of the
wasm-opt tool and reduces the wasm binary size of "hello world" by 14%.

Change-Id: Iba3043df05bf6aab4745c5f8015c0337fc218aff
Reviewed-on: https://go-review.googlesource.com/c/go/+/167801


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>