fix: make use_libtool_on_macos_setting public

[thesayyn]

#378 introduced this setting to allow users to disable libtool all together even on macOS, however this flag was not made public for toolchain implementors to condition on.

I have two different tool_maps that overrides the ar binary but it was conditioned on @platforms//os:macos like rules_cc was, but with the change @keith made, it changed the flags leading to broken ar action

# use libtool
cc_tool_map(
    name = "libtool_tools",
    tools = {
        "@rules_cc//cc/toolchains/actions:ar_actions": ":llvm-libtool-darwin",
    },
)

# use llvm-ar
cc_tool_map(
    name = "default_tools",
    tools =  {
        "@rules_cc//cc/toolchains/actions:ar_actions": ":llvm-ar",
    },
)

alias(
 name = "tools",
 actual = select({
      "@rules_cc//cc/toolchains/args/archiver_flags:use_libtool_on_macos_setting": "libtool_tools",
      "//conditions:default": "default_tools",
  })
)