Fix usage of legacy macro for declare_toolchain #26
Description
Currently, the declare_toolchain macro must use legacy macros. This is due to a naming oversight in rules_cc. cc_toolchain produces a target named _<name> which does not follow the naming convention required by symbolic macros.
ERROR: /home/mark/.cache/bazel/_bazel_mark/5979f8eda1b07b2dab85fbe51f06c597/external/toolchains_cc++cxx_toolchains+toolchains_cc_default_toolchain/BUILD:4:18: in cc_toolchain_config rule @@toolchains_cc++cxx_toolchains+toolchains_cc_default_toolchain//:_toolchains_cc_default_toolchain_cc_toolchain_config: Target @@toolchains_cc++cxx_toolchains+toolchains_cc_default_toolchain//:_toolchains_cc_default_toolchain_cc_toolchain_config declared in symbolic macro 'toolchains_cc_default_toolchain' violates macro naming rules and cannot be built. Name must be the same as the macro's name, or the macro's name followed by '_' (recommended), '-', or '.', and a non-empty string.