[CMake][BLAKE3] Fix unused -mavx512vl warning when CMAKE_OSX_ARCHITECTURES=arm64

This fixes
```
% ninja -C out/play LLVMSupport
ninja: Entering directory `out/play'
[151/158] Building ASM object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_avx512_x86-64_unix.S.o
clang: warning: argument unused during compilation: '-mavx512vl' [-Wunused-command-line-argument]
```

and applies `disable_blake3_x86_simd()`.

This fixes the root cause of commit 5160f6fe ("broke cross-builds of llvm from x86_64 to arm64 mac"...)