build.rs: only add -mno-avx when targeting x86_64

[chenrui333]

fixes #253

-mno-avx is x86_64 specific and since llvm@18, it fails with error: unsupported option

$ echo 'int main(){return 0;}' |
>   /opt/homebrew/opt/llvm\@17/bin/clang -x c - -c \
>     --target=arm64-apple-macosx -mno-avx -o /dev/null
clang: warning: argument unused during compilation: '-mno-avx' [-Wunused-command-line-argument]

$ echo 'int main(){return 0;}' |
  /opt/homebrew/opt/llvm\@18/bin/clang -x c - -c \
    --target=arm64-apple-macosx -mno-avx -o /dev/null
clang: error: unsupported option '-mno-avx' for target 'arm64-apple-macosx'

[chenrui333]

cc @dot-asm