Add override path for archiver_flags in rules based toolchain

[keith]

Currently this toolchain assumes that if you're targeting macOS you will always use llvm-libtool-darwin:

rules_cc/cc/toolchains/args/archiver_flags/BUILD

Line 21 in bd6844d

"@platforms//os:macos": ["-static"],

Technically this is not a requirement and using llvm-ar is fine. It seems like maybe this configuration decision should be more flexible than forcing llvm-libtool-darwin if you ever target macOS

[keith]

another practical reason to allow using ar on darwin is llvm-libtool-darwin warns about things like multiple object files with the same name in the archive, without that actually being an issue in most cases