Add support for solaris atomicops

This patch adds support for atomic operations on Solaris, on any platform.
It makes use of the atomic functions made available in Solaris' atomic.h
header.
I was suggesting putting yourself as the copyright owner and removing "Google Inc.", "Google Protocol Buffers" to  avoid confusion. Note that putting "Google Inc." here does not affect your rights.

Let me know if you want to use your own name, or is fine with using "Google Inc.".