ovs-atomics: Add atomic support Windows.

ovs-atomics: Add atomic support Windows.

Before this change (i.e., with pthread locks for atomics on Windows),
the benchmark for cmap and hmap was as follows:

$ ./tests/ovstest.exe test-cmap benchmark 10000000 3 1
Benchmarking with n=10000000, 3 threads, 1.00% mutations:
cmap insert:  61070 ms
cmap iterate:  2750 ms
cmap search:  14238 ms
cmap destroy:  8354 ms

hmap insert:   1701 ms
hmap iterate:   985 ms
hmap search:   3755 ms
hmap destroy:  1052 ms

After this change, the benchmark is as follows:
$ ./tests/ovstest.exe test-cmap benchmark 10000000 3 1
Benchmarking with n=10000000, 3 threads, 1.00% mutations:
cmap insert:   3666 ms
cmap iterate:   365 ms
cmap search:   2016 ms
cmap destroy:  1331 ms

hmap insert:   1495 ms
hmap iterate:  1026 ms
hmap search:   4167 ms
hmap destroy:  1046 ms

So there is clearly a big improvement for cmap.

But the correspondig test on Linux (with gcc 4.6) yeilds the following:

./tests/ovstest test-cmap benchmark 10000000 3 1
Benchmarking with n=10000000, 3 threads, 1.00% mutations:
cmap insert:   3917 ms
cmap iterate:   355 ms
cmap search:    871 ms
cmap destroy:  1158 ms

hmap insert:   1988 ms
hmap iterate:  1005 ms
hmap search:   5428 ms
hmap destroy:   980 ms

So for this particular test, except for "cmap search", Windows and
Linux have similar performance. Windows is around 2.5x slower in "cmap search"
compared to Linux. This has to be investigated.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
[With a lot of inputs and help from Jarno]
Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>