depend-cleanup.sh: Handle commit cbdec8db18b5

That commit changed libc to use the MI pdfork implementation, but with
an incremental build the object file for the pdfork.S stub lingers and
causes a linker error.

Cleaning the depend file is not enouch, so modify clean_deps() to remove
object files as well, and add a call to ensure that pdfork.*o is
cleaned. The new file is _pdfork.o.

Reported by: jhb
Reviewed by: emaste
Fixes: cbdec8db18b5 ("libc: Add pdfork to the list of interposed system calls")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33322