Disable more of AFDO for host modules

afdo.addDep would never add a depencency on a profile for a host
module, but afdoDepsMutator would still propagate AfdoRDeps to
dependencies, afdoMutator would still create afdo variants, and
afdo.flags would still add the -funique-internal-linkage-names
flag.  Exit early from all of these functions, since the afdo
variations will never be useful.

Test: afdo_test.go
Change-Id: I255fcbb86c99871916e571fbc74075d918b24745