Disable LIBCXX_ENABLE_ABI_LINKER_SCRIPT for static library builds.

This option is geared towards shared library builds and causes static
library builds to fail if not explicitly disabled.

This patch fixes PR27706: https://llvm.org/bugs/show_bug.cgi?id=27706

Thanks rgoodfel@isi.edu for the catch.

llvm-svn: 269585