Update README with benchmarks re-run on 64 bits OS
Update README with benchmarks re-run on 64 bits OS

The previous benchmarks were run on the same hw, which was then running
a 32 bits OS. Now it is running a 64 bits OS.  (Another difference is
also that the compilers and languge vm versions are a bit newer)

Generally, the new results show:

  * a 59% speedup for C++
  * a 4% slowdown for Python
  * a 15% speedup for Java

(I saw very similar figures for the OpenJDK and for the Oracle JVM)

mmmries/gpb