Parallelize building gnu/usr.bin/groff

This speeds up building the directory from the bootstrap-tools stage in
buildworld as well as building from the subdirectory

Based on a patch submitted via -arch:
https://lists.freebsd.org/pipermail/freebsd-arch/2014-December/016493.html

MFC after: 1 week
Submitted by: Jia-Shiun Li <jiashiun@gmail.com>
Sponsored by: EMC / Isilon Storage Division