Two changes to support building shared libraries that contain multiple

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22127 91177308-0d34-0410-b5e6-96231b3b80d8