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

[dot-asm]

Since the problem is demonstrably user-configuration-specific, the preferred course of action is to identify its root. Which is why the question was asked in #253 as opposed to rushing ahead with an dodge-the-problem fixes. Maybe there is more to it. With this in mind, would you answer the questions?