base: Use STL C++11 random number generation

This patch changes the random number generator from the in-house
Mersenne twister to an implementation relying entirely on C++11 STL.

The format for the checkpointing of the twister is simplified. As the
functionality was never used this should not matter. Note that this
patch does not actually make use of the checkpointing
functionality. As the random number generator is not thread safe, it
may be sensible to create one generator per thread, system, or even
object. Until this is decided the status quo is maintained in that no
generator state is part of the checkpoint.

src/base/SConscript

@@ -51,7 +51,6 @@ Source('misc.cc')
 Source('output.cc')
 Source('pollevent.cc')
 Source('random.cc')
-Source('random_mt.cc')
 if env['TARGET_ISA'] != 'null':
     Source('remote_gdb.cc')
 Source('socket.cc')