Migration to dynamic libs for llvm and clang

[Ed Maste]

One of goals for the toolchain prior to the FreeBSD 11 branch is to
create a libllvm.so and libclang.so for use by all of the LLVM family
tools installed in the base system. This message is just a heads-up in
case anyone has questions or comments on the idea.

We currently build a large number of static libs for the llvm and
clang components, which are reused in a number of tools in the LLVM
family. The resulting binaries end up quite large, and as a group
require a lot of disk space. For example, LLDB includes a copy of
Clang, used as its expression parser.  As a result, on my desktop
/usr/bin/clang and /usr/bin/lldb are both 27MB.

Over time we may add additional LLVM family tools (e.g.,
llvm-objdump), and this will help avoid excessive bloat as we do so.

I expect libllvm.so and libclang.so will go in /usr/lib/private.