[release-branch.go1.17] cmd/link: increase reserved space for passing env on wasm

On wasm, the wasm_exec.js helper passes the command line arguments and
environment variables via a reserved space in the wasm linear memory.
Increase this reserved space from 4096 to 8192 bytes so more environment
variables can fit into the limit.

Later, after https://golang.org/cl/350737 landed, we can switch to the
WASI interface for getting the arguments and environment. This would
remove the limit entirely.

Updates #49011.
Fixes #49154.

Change-Id: I48a6e952a97d33404ed692c98e9b49c5cd6b269b
Reviewed-on: https://go-review.googlesource.com/c/go/+/358194


Trust: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
(cherry picked from commit 252324e8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/359399
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>