[PATCH] config.sub: add javascript-unknown-ghcjs

[Adam Joseph]

GHC has been using a custom triple "javascript-unknown-ghcjs" for their (non
asm.js, non wasm) javascript-emitting kernel-less target.

  
https://gitlab.haskell.org/ghc/ghc/-/commit/6636b670233522f01d002c9b97827d00289dbf5c

This triple is a bit of an oddball, so the support for it is highly restricted
in order to discourage further proliferation of the javascript "cpu" or ghcjs
"operating system", which are valid only in combination with each other.

In the (quite large) corpus of software packaged for nixpkgs we've needed to
allow non-config.sub-accepted triples in only three situations [1]; this is one
of them.  We'd like to upstream this case to GNU config since GHC has already
shipped compilers that use it and they don't appear to be having any second
thoughts about it.

[1] https://github.com/NixOS/nixpkgs/pull/235230#discussion_r1295333268