Fix _mm_loadu_si32 detection for vendored Clang.
Apple uses different __clang_major__ version numbers than upstream, so our test did not work. This caused compilation failures with, e.g., XCode 10.1, which reports __clang_major__ as 10 despite being forked from upstream's 7.0 branch.
Fixes #2369 (closed)
Edited by Timothy B. Terriberry