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tools/build: Provide FreeBSD's bitstring API when cross-building

Description

tools/build: Provide FreeBSD's bitstring API when cross-building

This is needed for building makefs as a cross-tool since the ZFS code
uses these APIs.

Reviewed by: emaste
Fixes: 240afd8c1fcc ("makefs: Add ZFS support")
Differential Revision: https://reviews.freebsd.org/D36133