GHC cannot create output dir tree

```
mkdir tmp
ghc -c csrc/code.c -outputdir tmp/
# This works fine, creates tmp/csrc/code.o
wasm32-unknown-unknown-wasm-ghc -c csrc/code.c -outputdir tmp/
error: unable to open output file 'tmp/csrc/code.o': 'No such file or directory'
```
Due to this cannot compile cabal projects with c-sources.