target-finalize: libtread_db: support nonstd toolchain setups

Certain toolchain configs (E.G. on blackfin), don't use symlinks, so the
file name to match is the actual SONAME (.so.*). Support these toolchains
as well.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>