Create a Builder object for .isa files in arch/SConscript.

Create a Builder object for .isa files in arch/SConscript.
Start using SCons File objects to avoid fixed paths in
subordinate SConscripts.

SConscript:
    Push isa_parser stuff (including .isa scanner) down into
    arch/SConscript.
arch/SConscript:
    Create a Builder object for .isa files, including existing scanner.
    Return file objects generated by isa-specific SConscript
    back up to parent.
arch/alpha/SConscript:
arch/mips/SConscript:
arch/sparc/SConscript:
    Convert sources to scons File objects, so file names can be specified
    relative to the current directory.
    Invoke new builder for isa description, and get generated sources from
    there (instead of listing them explicitly).
arch/isa_parser.py:
    Get rid of third argument ("include_path").
    It was a pain to generate this from scons, and it turned out
    it's not needed anyway, since the only included file
    (decoder.hh) will be in the same directory as the sources.

--HG--
extra : convert_revision : 36861bcef36763f229704d8cb7a642b4486a3581