Add a helper function for rules that need the CC toolchain

[katre]

Currently, rules that require the CC toolchain via toolchain resolution use this syntax:

foo = rule(
    ...
    toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
)

(Possibly with other toolchain types present)

With optional toolchains, the syntax is changing, and will wait on a Bazel release.

We should provide a simple helper function that rules can use to be forward compatible when the new optional toolchains are release:

load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "use_cpp_toolchain")

foo = rule(
    ...
    toolchains = [other toolchains] + use_cpp_toolchain(mandatory = True),
)

The implemenation of use_cpp_toolchain will update with Bazel and will either return the bare-string toolchain type (as used currently) or the more complicated config.toolchain_type call. Rules won't need to make changes (beyond changing the value of mandatory if desired).

The only parameter will be mandatory, which will default to True for backwards compatibility.