Enable assembly for clangcl build on Windows.  Previously assembly was disabled so clangcl would work, but only with C code.  As clangcl mimics both Visual C and GCC, ifdefs need to pick one or the other or often you'll end up with both.  In this CL we disable most Visual C code and use the GCC versions which allow assembly for both 32 and 64 bit intel.
BUG=412
TESTED=clang=1 build on windows
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/51389004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1341 16f28f9a-4ce2-e073-06de-1de4eb20be90