llvm-3.9: move shared libLLVM to a separate output

Fixes #18840: too large closure of mesa_drivers.

Tested atop 16.09:
 - clang compiles a hello-world app;
 - mesa seems to link OK;
 - ispc builds.

Size comparison:
 - 80 MB of full llvm-3.7 on 16.03;
 - 200 MB of full llvm-3.9 on 16.09 before this patch;
 - 50 MB of libLLVM after this commit.