libcxxrt: define SHLIBDIR before including bsd.own.mk

Otherwise bsd.own.mk overrides it, causing libcxxrt.so.1 to be
erroneously installed into /usr/lib.

Also add an ObsoleteFiles.inc entry, for removing the bad copy.

Reported by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Fixes: da77a1b4f0df