rtld-libc: fix incremental build

ar cr is an update of an archive, not a creation of a new one. During
incremental builds (e.g. with meta mode) the archive was not getting
cleaned, and so could retain now-deleted objects from previous builds.
Now, delete the archive before creating/updating it.

Reviewed by: arichardson, bdrewery, kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27663