Two changes to support building shared libraries that contain multiple

llvm archive or re-linked libraries:
1. Permit the "JIT" special keyword on LLVMLIBS to be recognized when
   building a library, not just for building tools
2. If LINK_LIBS_IN_SHARED is set, the LLVMLIBS and USEDLIBS can be specified
   when linking a shared library and the libraries listed will be
   incorported into the shared library. THis is only used when the
   SHARED_LIBRARY variable is set.

llvm-svn: 22127