Add flags variants to linker_files / stack(9) symbol resolution

Some best-effort consumers may find trylock behavior for stack(9) symbol
resolution acceptable. Expose that behavior to such consumers.

This API is ugly. If in the future the modules and linker file list locking
is cleaned up such that the linker_files list can be iterated safely without
acquiring a sleepable lock, this API should be removed. However, most of
the time nothing will be holding the linker files lock exclusive and the
acquisition can proceed.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17620