Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Update build glue for libclang_rt.

Next step in updating llvm/clang build glue: make libclang_rt build.

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.