cc_toolchain's tool_map should be cfg-exec-configured

[dzbarsky]

However, it's not enough for tool_map to be on cc_toolchain_config - it must be on the toolchain itself to properly share the same exec platform as the action requesting the toolchain. Therefore we rearrange the implementation of the cc_toolchain macro to pass it to the rule, and teach the rule to accept it from either the attribute or the cc_toolchain_config. Eventually, cc_toolchain_config can be retired, though it will be a long journey.