Fix libc rebuild support

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
external-sourcery: ensure the multilib link is created

This splits out the creation of the link(s) from the libc recipes to avoid
duplicating the logic, for now.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
eglibc*: split out the multilib link bits to a .inc

Signed-off-by: Christopher Larson <chris_larson@mentor.com>