atomicops-internals-windows.h: Don't declare intrincs when using Clang

Clang provides inline definitions in its Intrin.h file. If we declare our
own intrinsic functions in base::subtle::, we will use those instead,
and fail to link because they are not defined anywhere.

BUG=82385

Review URL: https://codereview.chromium.org/150823003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248240 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248240