Build system cleanups for the compiler.

main.c:
    Delete this - it was only ever needed for the GCC backend.

compiler/Mmakefile:
    Delete targets for building the compiler as a library --
    this was only required by the GCC backend.

    Don't link against ../main.o.

    Delete rules for preprocessing .pp files.
    We don't currently have any files that need to be preprocssed
    and this particular Mmakefile is complicated enough with
    additional clutter in it.

tools/bootcheck.m:
    Conform to the removal of main.c

    Don't set GCC_SRC_DIR in the stage 2 Mmake.params file.