CPU: Unify initMemProxies across CPUs and simulation modes

CPU: Unify initMemProxies across CPUs and simulation modes

This patch unifies where initMemProxies is called, in the init()
method of each BaseCPU subclass, before TheISA::initCPU is
called. Moreover, it also ensures that initMemProxies is called in
both full-system and syscall-emulation mode, thus unifying also across
the modes. An additional check is added in the ThreadState to ensure
that initMemProxies is only called once.