libtool: fix contaminated path to lt_truncate_bin

libtool: fix contaminated path to lt_truncate_bin

lt_truncate_bin path is contaminated by the path from the sysroot
directory for the build host.

Steps to reproduce this issue:
$ bitbake -c cleanall libtool
$ bitbake coreutils-native
$ bitbake libtool

$ grep -in "lt_truncate_bin=" tmp/work/*/libtool/*/image/usr/bin/libtool

Signed-off-by: Alexandru Moise <alexandru.moise@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>