[libc++] Fix linker script generation

Handle the case when libc++abi and libunwind are being built together
with libc++ in the runtimes build. This logic was used in the previous
implementation but dropped in r374116.

Differential Revision: https://reviews.llvm.org/D68791

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374510 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed