Add support for sampled PC profiling to FullCPU.

Add support for sampled PC profiling to FullCPU.
Simple text list of symbol (or address) and count
will be dumped to m5prof.<cpu-name> if the cpu's
pc_sample_interval param is set.

SConscript:
    Add cpu/full_cpu/pc_sample_profile.cc
base/callback.hh:
    Add a comment about MakeCallback.
    Fix type in another comment.
base/loader/symtab.cc:
    Revamp findNearestSymbol() to provide addresses of both
    nearest symbols (preceding and following) as well as
    string for former.

    Move global definition of debugSymbolTable here too.
base/loader/symtab.hh:
    Revamp findNearestSymbol() to provide addresses of both
    nearest symbols (preceding and following) as well as
    string for former.

    Move global declaration of debugSymbolTable here too.
cpu/exetrace.cc:
    Use new findNearestSymbol() interface for trace symbols.
kern/linux/linux_system.cc:
sim/system.cc:
    Remove extern of debugSymbolTable (now in symtab.hh)
sim/process.cc:
    Initialize debugSymbolTable if binary has a symbol table.

--HG--
extra : convert_revision : 0b5393dc39c40ac88c953684708f1125da550671