Either
nim c --passC:"-fno-asynchronous-unwind-tables -fno-exceptions -fno-rtti -fverbose-asm -masm=intel -S" -d:release --noLinking:on "path/to/file.nim"
The result will be in $HOME/.cache/file_r/file.c.o
(_r for release)
The C sources or Nim sources (with --debugger:native) will be interleaved with ASM
Otherwise use:
nim c --passC:"-fno-asynchronous-unwind-tables -fno-exceptions -fno-rtti -masm=intel -S" -d:release --noLinking:on "path/to/file.nim"
i.e. remove -fverbose-asm, for pure ASM output.