depend-cleanup.sh: Handle commit cbdec8db18b5

[markj]

Summary

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
Fixes: cbdec8db18b5 ("libc: Add pdfork to the list of interposed system calls")
Sponsored by: The FreeBSD Foundation

Test Plan

$ cd lib/libc
$ git revert -n cbdec8db18b5
$ make cleandir
$ make -s WITHOUT_TESTS=
$ git checkout -f
$ make -s WITHOUT_TESTS= # fails
$ cd ../../
$ sh tools/build/depend-cleanup.sh /usr/obj/usr/home/markj/src/freebsd/amd64.amd64
Removing stale dependencies for pdfork.S
$ sh tools/build/depend-cleanup.sh /usr/obj/usr/home/markj/src/freebsd/amd64.amd64
$ cd lib/libc
$ make -s WITHOUT_TESTS= # succeeds

[emaste]

tools/build/depend-cleanup.sh

33–34

Might want to make this "stale dependencies and objects for" now