image-mklibs: pass correct libdir to mklibs

image-mklibs: pass correct libdir to mklibs

libdir should be specified, or else mklibs won't work for 64bit targets.
It wouldn't be able to find the libs.

    Traceback (most recent call last):
      File "<build>/bitbake_build/tmp/sysroots/i686-linux/usr/bin/x86_64-wrs-linux/mklibs", line 553, in <module>
        header = elf_header(find_lib(libraries.copy().pop()))
      File "<build>/bitbake_build/tmp/sysroots/i686-linux/usr/bin/x86_64-wrs-linux/mklibs", line 89, in elf_header
        raise Exception("Cannot find lib: " + obj)
    Exception: Cannot find lib:

Signed-off-by: Jesse Zhang <sen.zhang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>