One free multiple target cross assembler I have used on few occasions in the past is SB-Projects SB-Assembler. SB-Assembler 3 is written in Python3 and runs on Linux, MAC and Windows machines.
https://www.sbprojects.net/sbasm/ https://www.sbprojects.net/sbasm/crosses.php https://www.sbprojects.net/sbasm/download.php https://github.com/sbprojects/sbasm3 The syntax it uses might need some translation from the syntax used by other tools. Other than that I found it useful for what I needed when I used it. Also, I don't think there is any linker / librarian support, just a single output file, which probably rules it out for large projects.
