package_rpm: enhance strip_multilib function

package_rpm: enhance strip_multilib function

The previous logic could only strip MLPREFIX from the first element in
an array, take an example, strip_multilib the [lib32-a lib32-b lib32-c]
will result in [a lib32-b lib32-c]. We need to strip prefix one by one.

This commit change the logic to strip all elements' multilib prefix.

[YOCTO #1368]

CC: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>