Missing platform support for m68k architecture

[glaubitz]

The architecture Motorola 68000 (m68k) is currently not supported, the build fails with:

../src/tup/platform.c:74:2: error: #error Unsupported cpu architecture. Please add support in tup/platform.c
   74 | #error Unsupported cpu architecture. Please add support in tup/platform.c
      |  ^~~~~

Full build log available here.