tools/build: stage stdckdint.h's dependencies for non-FreeBSD hosts

37bd69d43c7 gave stdckdint.h two new includes, <sys/_visible.h> and
<sys/ckdint.h>. Neither reaches a non-FreeBSD host: _visible.h is
staged only under ${.MAKE.OS} == "FreeBSD" and ckdint.h is not staged at
all, so the libc bootstrap fails on reallocarray.o when cross-building
from macOS. Both headers are self-contained; stage them alongside
stdckdint.h.

Fixes: 37bd69d43c7 ("sys: Add sys/ckdint.h")
Reviewed by: rpaulo, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58943