[CMake] Fix in-tree libcxxabi build support after r283659

r283659 changed the argument to gen_link_script.py from SCRIPT_ABI_LIBNAME to
LIBCXX_LIBRARIES_PUBLIC, assuming that all of the items in the
LIBCXX_LIBRARIES_PUBLIC list were library names. This is not right, however,
for in-tree libcxxabi builds, we might have the target name in this list. There
was special logic to fixup SCRIPT_ABI_LIBNAME for this situation; change it to
apply a similar fixup for LIBCXX_LIBRARIES_PUBLIC.

llvm-svn: 283684