Fix the ELF shared library build targets

 lldELF is used by each ELF backend.  lldELF's ELFLinkingContext
 also held a reference to each backend, creating a link-time
 cycle.  This patch moves the backend references to lldDriver.

 Differential Revision: http://reviews.llvm.org/D7119

llvm-svn: 226922