Delete the MLDS->GCC (assembler) backend.

As discussed in the recent Mercury meeting, remove support for the GCC backend.
It was very much out of date and supporting it proprerly would means having to
track changes to GCC's internals.  Furthermore, its presence complicates
building the compiler.

The main thing this change does not address is the fact that we invoke
the compiler through C code, e.g. main.c in the top-level of the source
tree.  This was required by the GCC backend and can now be removed, but
I will do that as a separate change.

configure.ac:
Mmake.common.in:
scripts/Mmake.rules:
compiler/Mercury.options:
compiler/Mmakefile:

compiler/gcc.m:
compiler/maybe_mlds_to_gcc.pp:
compiler/mlds_to_gcc.m:
    Delete the files containing the GCC backend.

compiler/options.m:
compiler/handle_options.m:
    Delete support for `--target asm' and `--pic'.
    (The latter was only used by the GCC backend.)

compiler/*.m:
doc/user_guide.texi:
compiler/notes/comiler_design.html:
compiler/notes/work_in_progress.m:
    Conform to the above change.

README.gcc-backend.m:
    Delete this file.
Update tools directory to account for removal of GCC backend.

tools/binary:
tools/unary:
tools/bootcheck.m:
	As above.