A change in upstream Emscripten 1.39.1+ made our buildsystem error
out where it was previously only issuing a warning:
```
[ 5%] Linking Static Library ==> main/libmain.javascript.opt.bc
shared:WARNING: Assuming object file output in the absence of `-c`, based on output filename. Please add with `-c` or `-r` to avoid this warning
Ranlib Library ==> main/libmain.javascript.opt.bc
/opt/emsdk/upstream/bin/llvm-ranlib: error: unable to load 'main/libmain.javascript.opt.bc': file too small to be an archive
```
As advised on emscripten-core/emscripten#9806, we should be using
`emar` here to create the static library and not `emcc`.
This was apparently done to workaround Emscripten issues in the past,
but evidently this is no longer necessary.
The rest of the `env` redefinitions should probably be re-assessed
against the current state of Emscripten.
Fixes#33374.
e9e2a4b044