When using the compiled .wasm with misc/wasm/wasm_exec.js, we get an error message if the site prohibits eval() via the Content-Security-Policy header. This can be resolved by moving the callback helper code from src/syscall/js/callback.go to misc/wasm/wasm_exec.js.

Fixes #26748

Change-Id: I28f271b8a00631f4c66a1ac31305e85f20f9d420
GitHub-Last-Rev: a6a0268f38d36c198ca6b4ceb2e75cc8afec74eb
GitHub-Pull-Request: golang/go#26750
Reviewed-on: https://go-review.googlesource.com/127296Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>