JS: replace "js" architecture with "javascript"

Despite Cabal supporting any architecture name, `cabal --check` only
supports a few built-in ones. Sadly `cabal --check` is used by Hackage
hence using any non built-in name in a package (e.g. `arch(js)`) is
rejected and the package is prevented from being uploaded on Hackage.

Luckily built-in support for the `javascript` architecture was added for
GHCJS a while ago. In order to allow newer `base` to be uploaded on
Hackage we make the switch from `js` to `javascript` architecture.

Fixes #22740.

Co-authored-by: Ben Gamari <ben@smart-cactus.org>