Now compiler-rt has been updated in r276851, bring in the various
sanitizer libraries that already work on FreeBSD:

• asan: Address Sanitizer
• ubsan: Undefined Behavior Sanitizer
• profile: Profile Guided Optimization support

Please note that these libraries are *experimental* at this stage, so
the main Makefile is not yet connected to the build.

Since I didn't want to needlessly edit BSD.usr.dist, you will also have
to create the install directory /usr/lib/clang/3.5.0/lib/freebsd
manually for now.