tools/build: Always bootstrap (sys/)bitstring.h

Older, supported FreeBSD versions lack bit_ntest() so hoist the boostrap
installation out of the OS!=FreeBSD case and always install it. A more
precise criteria is possible, but would add little value.

This fixes bootstrapping makefs as a build tool on 13.1.

Reviewed by: jrtc27, emaste
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D37951