misc/wasm: fix permissions on wasm_exec.js

Currently wasm_exec.js is executable (0755) yet has no interpreter.
Indeed wasm_exec.js is only ever used as an argument to Node or loaded
via a <script> tag in a browser-loaded HTML file.  Hence the execute
mode bits are superfluous and simply serve to clutter your PATH if
$GOROOT/misc/wasm is on your PATH (as is required if you want to run go
test syscall/js).

Change-Id: I279e2457094f8a12b9bf380ad7f1a9f47b22fc96
Reviewed-on: https://go-review.googlesource.com/120435
Run-TryBot: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>