Add debug-test-64-clang Configure target

Apparently clang 3.1, which ships with FreeBSD 9.1, completely ignores
-fsanitize. Clang 3.3, which ships with FreeBSD 9.2, compiles with it, but
fails to link due to the absence of libasan:

http://lists.freebsd.org/pipermail/freebsd-hackers/2013-December/043995.html
https://www.mail-archive.com/cfe-commits@cs.uiuc.edu/msg92260.html
http://reviews.llvm.org/D2644