Looks like Tandy used a Z80 cross assembler/linker called ALDS written
in C to build the software for their Z80 based systems. This development
tools first ran on a VAX under VMS and later then under BSD UNIX.

This ALDS now is open source and it can be build on various UNIX systems
and used as cross development tools for Z80 based systems. Also the
assembler was enhanced to support the HD64180/Z180 processor.

Home page of ALDS:

Udo Munk
The real fun is building it and then using it...