compcert: clean up expression

  - Require Coq 8.6.1+
  - Split substituteInPlace call into patchPhase
  - Constrain platforms correctly to x86_64 Linux/Darwin, which was all
it supported anyway (there was no way to properly configure i686 builds,
nor cross builds. In the future there might be)
  - Minor stylistic cleanups
  - Add new 'man' and 'doc' outputs (the previous attempt to move the
build artifact outputs into $lib no longer worked correctly and they
were installed into 'out' instead, this fixes it completely).
  - Clean up weird binary artifacts left in $out (that were already
in $lib)
  - Wrap ccomp to undefine _FORTIFY_SOURCE; otherwise it causes
annoying warnings on every invocation

Signed-off-by: Austin Seipp <aseipp@pobox.com>