bsd.compat.mk: Honour XSTRIPBIN for lib32

Currently we only honour XNM and XOBJCOPY, but XSTRIPBIN is important
during install. Otherwise we end up using STRIPBIN for the host, not the
target, which is normally the same, but may not be, especially on
non-FreeBSD systems. In particular, cheribuild will build FreeBSD with
STRIPBIN=/usr/bin/strip XSTRIPBIN=strip (with the latter referring to
the bootstrap strip in PATH), which breaks for WITH_LIB32 when the
host's /usr/bin/strip is unable to process the lib32 binaries (e.g.
building arm64 FreeBSD's lib32 on an amd64 GNU/Linux system).

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