libexec: Automatically generate rtld-elf list and generalise TAGS

Note that the pattern for matching is made slightly more specific, so as
to permit libcompats where one is a prefix of another (e.g. CheriBSD has
lib64 and lib64c).

Reviewed by: brooks, jhb, emaste, imp, kib
Differential Revision: https://reviews.freebsd.org/D41183