New loader structure. Expand Nate's ObjectFile to automatically detect file formats

New loader structure.  Expand Nate's ObjectFile to automatically detect file formats
and generate an appropriate child object, which knows how to load its text & data into memory
(and symbols, for ecoff only at this point).  Ecoff format supports Tru64 binaries, kernel, and
console.  A.out format is used by PAL code.  Elf support is there and appears to work for
Alpha/Linux binaries, but awaits a Linux syscall emulation layer before further testing.

arch/alpha/ecoff_machdep.h:
base/coff_sym.h:
base/coff_symconst.h:
base/exec_ecoff.h:
    Add Id string & provenance comment, clean up types
base/object_file.cc:
base/object_file.hh:
    Add auto format detection and text/data loading.
sim/prog.cc:
    Moved binary loading & stack setup here (was in arch/alpha/loader.cc).  Some of this is
    platform-dependent, but I think most of it is not.  We'll factor out the platform-dependent
    parts when the need arises.
sim/prog.hh:
    Get rid of unused environ_base field.
sim/system.cc:
    Use new ObjectFile loader structure for console, kernel, & palcode.

--HG--
extra : convert_revision : 8eae509bf71cf204bc3ec78c68699cfd01baed97