misc/wasm: fix passing large negative integers from JS to Go

This commit addresses a FIXME left in the code of wasm_exec.js to
properly get the upper 32 bit of a JS number to be stored as an
64-bit integer. A bitshift operation is not possible, because in
JavaScript bitshift operations only operate on the lower 32 bits.

Change-Id: I8f627fd604e592682d9d322942a4852db64a7f66
Reviewed-on: https://go-review.googlesource.com/113076


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>