It's possible to have files with odd permissions in the tmproot (or sysroot), causing rm to prompt for each one. Add -f to forcibly delete these.
Example:
override r--r--r-- emaste/wheel uarch for /usr/obj/usr/home/emaste/src/freebsd-git/wipbsd/amd64.amd64/sysroot/usr/include/c++/v1/__functional_base? y override r--r--r-- emaste/wheel uarch for /usr/obj/usr/home/emaste/src/freebsd-git/wipbsd/amd64.amd64/sysroot/usr/include/c++/v1/__libcpp_version? y override r--r--r-- emaste/wheel uarch for /usr/obj/usr/home/emaste/src/freebsd-git/wipbsd/amd64.amd64/sysroot/usr/include/c++/v1/__nullptr? y override r--r--r-- emaste/wheel uarch for /usr/obj/usr/home/emaste/src/freebsd-git/wipbsd/amd64.amd64/sysroot/usr/include/c++/v1/__string? y