Revert "[Support/BLAKE3] Re-enable building with the simd-optimized implementations"

This reverts commit 23519d30.

This breaks the build with clang-5:

/usr/bin/clang-5.0 -DBUILD_EXAMPLES -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Ilib/Support/BLAKE3 -Illvm/lib/Support/BLAKE3 -Iinclude -Illvm/include -fPIC -O3 -DNDEBUG -MD -MT lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_avx512_x86-64_unix.S.o -MF lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_avx512_x86-64_unix.S.o.d -o lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_avx512_x86-64_unix.S.o -c llvm/lib/Support/BLAKE3/blake3_avx512_x86-64_unix.S
...
llvm/lib/Support/BLAKE3/blake3_avx512_x86-64_unix.S:54:9: error: instruction requires: AVX-512 ISA
        kmovw k1, r9d
        ^