Add an optimized 32-bit implementation of the NIST P-256 elliptic curve.

ecp_256_32.c was written by Adam Langley and was reviewed in
https://codereview.chromium.org/10944017/

R=agl@chromium.org,rsleevi@chromium.org
BUG=172178
TEST=no build or test failures on iOS, Mac, and Windows

Review URL: https://chromiumcodereview.appspot.com/12042100

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/nss@179208 4ff67af0-8c30-449e-8e8b-ad334ec8d88c