cc: Make cc_genrule defaultable

Allows genrule_defaults to be used with cc_genrule.

This change does not create a cc_genrule_defaults, that can be done in a
follow-up change.

Bug: 325444956
Test: m [custom cc_genrule with defaults]
Change-Id: I9e2d26b31e04eb972801421d25b1ff6c8ef75369