Allow stdlib to be built and depended on with --features=race (#1317)

Passing bootstrap = True to go_rule now only controls whether _stdlib
and _builders attributes are added. stdlib continues to do this.

The presence or absence of _builders is no longer used to force off
race mode. This prevented stdlib from being built in race mode without
the aspect. Now, go_tool_binary (the only other bootstrap rule) has a
special _race = "off" attribute, which get_mode looks for.

Fixes #1316