Choose host/target ldconfig based on availability

Decide whether to use the host or target ldconfig based on an
availability check instead of internal/external toolchain selection.  An
external toolchain may very well provide an ldconfig while the host's
one may fail.
External toolchain generated by Gentoo crossdev:

$ LC_ALL=C i686-pc-linux-uclibc-ldconfig -r output/target/; echo $?
i686-pc-linux-uclibc-ldconfig: skipping /usr/lib: No such file or directory
0

vs. the host (x86_64-pc-linux-gnu) version:

$ LC_ALL=C /sbin/ldconfig -r output/target/; echo $?
/sbin/ldconfig: Can't open configuration file
output/target/etc/ld.so.conf: No such file or directory
/sbin/ldconfig: Can't open cache file /var/cache/ldconfig/aux-cache
: No such file or directory
1

Signed-off-by: Malte Starostik <m-starostik@versanet.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>