No longer install sys/nv.h and sys/cnv.h in lib/libnv/Makefile

Use tools/build/Makefile to install the headers into ${WORLDTMP}/legacy
instead. Compared to r336026 this has the minor advantage that it avoids
unncessary header installation when building the non-bootstrap libnv.

Reviewed By: bdrewery, kevans
Approved By: brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D16187