assume __clang_major__==0 has the latest version of
_mm256_broadcastsi128_si256. fixes builds with custom clang toolchains.

cherry-picked from libvpx:
33aef48f2 vpx_subpixel_8t_intrin_avx2: tolerate unversioned clang

BUG=b/30970831

Change-Id: I90becd56278e4716bd46e2ba9d910af977e8dfa6