> Never used Nim before so I might be doing something wrong though.

With Nim on weird targets you usually want:

- OS target = any

- Memory Management = ARC

- malloc instead of default Nim allocator

- turn off signal handler (if not POSIX)

- disable threads (most of the time)

Then look at how C is compiled and copy all compiler+linker flags to your Nim configuration. Here's an absolute minimal `config.nims` I used to compile Nim for C64 with LLVM-MOS[1] toolchain:

    import std/strutils
    
    --cc:clang
    --clang.exe:"mos-c64-clang"
    
    --os:any
    --cpu:avr
    --mm:arc
    
    --threads:off
    --define:usemalloc
    --define:noSignalHandler
    
    let args = [
      "-isystem $PWD/../mos-platform/c64/include",
      "-I$PWD/../mos-platform/c64/asminc",
      "-L$PWD/../mos-platform/c64/lib",
      "-mlto-zp=110",
      "-D__C64__",
    
      "-isystem $PWD/../mos-platform/commodore/include",
      "-I$PWD/../mos-platform/commodore/asminc",
      "-L$PWD/../mos-platform/commodore/lib",
      "-D__CBM__",
    
      "-isystem $PWD/../mos-platform/common/include",
      "-I$PWD/../mos-platform/common/asminc",
      "-L$PWD/../mos-platform/common/lib",
      "--target=mos",
      "-flto",
      "-Os",
      ].join(" ")
    
    switch("passC", args)
    switch("passL", args)

Nim side was easy, because I have already compiled Nim to WASM at that point and the flags are similar. Hard part was figuring out the C compiler flags: e.g. cmake structure and why compiler complains about missing symbols, when they're not missing (answer: include/lib order is very important).

[1] https://github.com/llvm-mos/llvm-mos