As with others, all I wanted is a starting point. Hopefully this helps someone else out.
The 8051 port described on FreeRTOS site and available in the Cygnal Demo folder, I have been able to confirm FreeRTOS V2.4.1 compiles with SDCC 2.5.0 May 8 2005 without error, zero modifications to the source and the Makefile as is. End up with an Intel hex file.
Just need the path added for your Make and SDCC binary. For completeness, GNU Make version used was 64 bit 4.3 on W10 64 bit.
Don’t know if the binary works, just that it compiles. At least a starting point. Assuming you can find the two versions commented on.