__strnlen_chk: avoid recursive strlen calls

Don't use FORTIFY_SOURCE on functions which implement
FORTIFY_SOURCE.

Bug: 12216860
Change-Id: I61db1b47ccdd6bdcf41eab3303f4806494016199
Clean up more recursive FORTIFY_SOURCE calls

Don't use FORTIFY_SOURCE on functions which implement
FORTIFY_SOURCE, to avoid infinite recursion problems.

The previous patch only addressed one of the problems.

Bug: 12216860
Change-Id: I6f30ae7cb5b481be9942add18182ea4839d348a6