Consider defaulting to ar command for static linking, instead of llvm-ar

[keynmol]

Currently we hard request llvm-ar and corresponding LLVM_BIN installation to be available.
This causes friction in environments where we use the globally installed clang.

ar command can do the same things (with object files, not LLVM bitcode), but it's available by default.

I propose we try to use ar if llvm-ar is not available.

https://sourcegraph.com/github.com/scala-native/scala-native/-/blob/tools/src/main/scala/scala/scalanative/build/LLVM.scala?L255:37