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Natively, all Watcom C/C++ compilers can produce is OMF (.obj) files by default. These can be linked with the Watcom linker into an ELF file but not plain binary. Its reccomended you use some other linker.
This is not exactly accurate. WLINK can link multiple OBJs into a flat binary and its possible to use a linker script to manually order and locate segments. There is also a sub-project of the assembler called JWASM that can produce flat binaries from ASM code.