libnv: Build PIC

Build libnv as position independent code so we can use it from shared
libraries.

MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29561

(cherry picked from commit 0dd13c77432ade1ae94c9661cbad5537e3e6ab1d)