oeqa/multilib: rework to use readelf from the host

It is difficult and error-prone to ensure binutils gets
installed into target images where this test may run;
on the other hand readelf is always present on the
host, as it is a part of HOSTTOOLS.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>