Note that the 'C' preprocessor is essentially independent of the 'C' language - so you can use it to prepare pretty much any text file - including assembler files.

I once worked on a project that used the 'C' preprocessor to create the Linker command file...

Most compilers either supply the preprocessor as a separate tool, or have an option to "preprocess only"

Since it's independent of the language, you don't even need an 8051-specific preprocessor; eg, you could use the one from the (free) Borland compiler.

I once worked on a (different) project that used the Borland preprocessor, because the cross-compiler's preprocessor was rubbish!