The BSD-style license that Go uses requires the license to be included
with the source distribution; so add it as LICENSE.wasmexec (to avoid
confusion with the other licenses in rclone) and note the location of
the license in wasm_exec.js itself.
Solves link error while running rclone's wasm version. Go's `walltime1` function was renamed to `walltime`. This commit updates wasm_exec.js with the new name.