Serialize the symbol tables

Serialize the symbol tables

base/loader/symtab.cc:
    Add support for clearing out the symbol table
    Add support for serializing the symbol table (clear on unserialize)
    Don't allow empty symbols to be entered into the table
base/loader/symtab.hh:
    Add support for clearing out the symbol table
    Add support for serializing the symbol table
sim/system.cc:
    Serialize the kernel, console, and palcode symbol tables so that
    we can capture any dynamic symbols that are added and so that we
    don't have to have the same kernel binary around to get the
    symbols right

--HG--
extra : convert_revision : 779888c88aa530f3adcd37dc7600a335951d05f7