Minor cleanup from building & diffing behavior

Minor cleanup from building & diffing behavior
on various platforms.

base/hashmap.hh:
    gcc on Alpha doesn't always define __LP64__,
    even though it arguably should.
cpu/exec_context.cc:
    Clear register file on non-full-system too (even though
    it typically gets overwritten by the initial regs from
    the Process object).
sim/process.cc:
    Clear initial register copy in Process object.
    Not all regs get initialized when the executable is loaded.

--HG--
extra : convert_revision : f1fe4734a5ea81331d70994cb5284b1e9db0dceb