misc/wasm: make sure sp is unsigned

An i32 passed from WebAssembly to JavaScript is always read as a signed
integer. Use the bitshift operator to turn it into an unsigned integer.

Fixes #40923

Change-Id: Ia91ed2145dd2fc3071e2fc22b86ebfcb3c1e9f4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/261358


Trust: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>