rc.d/ldconfig: Compute ldconfig paths in a function

Move logic that computes paths passed to ldconfig(8) to a
ldconfig_paths() function that can be called for multiple ABIs.

Reviewed by: olce, kib
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D44751

(cherry picked from commit e6e38bc522e29de6299536b547bf11dab11e9679)