Add ROADMAP.

TODO
  - setjmp
  - possibly: commented-out HAVE_FLOAT? in tccgen.c
  - integrate ARM ./bootstrap.sh build into commencement.scm
    + build gcc-2.95.3 ...etc, etc.

DONE:

  - ./pre-inst-env guix build --system=armhf-linux -e tcc-boot
  - mes-tcc can be bootstrapped, build and and rebuild itself, until
    boot6-tcc, with long longs and floats