Flattening and syscallReturn fixes

Flattening and syscallReturn fixes

src/cpu/o3/thread_context_impl.hh:
    Use flattened indices
src/cpu/simple_thread.hh:
    Use flattened indices, and pass a thread context to setSyscallReturn rather than a register file.
src/cpu/thread_context.hh:
    The SyscallReturn class is no longer in arch/syscallreturn.hh

--HG--
extra : convert_revision : ed84bb8ac5ef0774526ecd0d7270b0c60cd3708e