[CMake] Fix libc++abi standalone cmake build.

The cmake files install directory has been changed to
${prefix}/lib/cmake/llvm since r259821.  Searching cmake modules in
${prefix}/share/llvm/cmake will result in fatal errors.

This commit fixes the out-of-tree build by changing the CMake module
search path to: "$(llvm-config --obj-root)/lib/cmake/llvm"

llvm-svn: 283100