_tzcnt_u32() is not exported by clang-cl intrin.h so check for
clang-cl and enable an alterate for _tzcnt_u32()

Some discussions:
http://lists.llvm.org/pipermail/cfe-dev/2016-October/051329.html
https://bugs.llvm.org/show_bug.cgi?id=30506

TEST=Build with clang-cl