Auto merge of #37775 - alexcrichton:try-fix-dox, r=brson

rustbuild: Tweak default rule inclusion

If a rule is flagged with `default(true)` then the pseudo-rule `default:foo`
will include that. If a rule is also flagged with `.host(true)`, however, then
the rule shouldn't be included for targets that aren't in the host array. This
adds a filter to ensure we don't pull in host rules for targets by accident.