runtime, cmd/link: optimize memory allocation on wasm

WebAssembly's memory is contiguous. Allocating memory at a high address
also allocates all memory up to that address. This change reduces
the initial memory allocated on wasm from 1GB to 16MB by using multiple
heap arenas and reducing the size of a heap arena.

Fixes #27462.

Change-Id: Ic941e6edcadd411e65a14cb2f9fd6c8eae02fc7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/170950


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>