Attempt to let skia's SSE files build with clang-cl.

Attempt to let skia's SSE files build with clang-cl.

cl.exe can use intrinsics (like e.g. _mm_cvtsi128_si32()) even if arch
targetted by the compiler doesn't support the SSE level needed by the intrinsic.

clang (and gcc) can't do this, because it converts intrinsics into general
LLVM operations, and the LLVM bitcode is then translated back into assembly
later on, based on the target arch.

So explicitly enable SSSE3 and SSE4 when using clang-cl, so that the intrinsics
compile.

(It's unfortunate that clang-cl deviates fairly heavily from cl in this regard.
But this is very difficult to change, and it's the only larger deviation so
far.)

BUG=82385
R=rnk@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289735

[Reid Kleckner, 2014-08-15 00:40:03]

lgtm

Thanks, I started this bug I got stuck somewhere in libvpx because their gyp is
generated from a shell script that can't be run on Windows.

https://codereview.chromium.org/475273002/diff/1/skia/skia_library_opts.gyp
File skia/skia_library_opts.gyp (right):

https://codereview.chromium.org/475273002/diff/1/skia/skia_library_opts.gyp#n...
skia/skia_library_opts.gyp:9: # Due to an unfortunate intersection of lameness
between gcc and gyp,
I feel guilty for propagating this "lameness" to clang.  Oh well. :)

[Nico, 2014-08-15 00:45:23]

Committed patchset #1 manually as 289735 (presubmit successful).