installworld: handle ldd including preloaded objects

The installworld target makes a temporary copy of binaries to be used
during the install.  Libraries that they depend on are also included,
found by using `ldd`.

After commit 0913953c9e ldd started listing preloaded objects,
including [vdso], under a [preloaded] header.  Skip ldd output that is
enclosed in square brackets.

Reviewed by:	cy, kib [earlier version]
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D34734

(cherry picked from commit b3b462229f)
(cherry picked from commit e10026a406)

Approved by:	re (gjb)

Makefile.inc1

@@ -1383,6 +1383,10 @@ distributeworld installworld stageworld: _installcheck_world .PHONY
 	if [ -z "${CROSSBUILD_HOST}" ] ; then \
 		libs=$$(ldd -f "%o %p\n" -f "%o %p\n" $$progs 2>/dev/null | sort -u | \
 		    while read line; do \
+			case $$line in \
+			"["*"]") \
+				continue;; \
+			esac; \
 			set -- $$line; \
 			if [ "$$2 $$3" != "not found" ]; then \
 				echo $$2; \