[libcxx] Make libc++.so a linker script by default on most platforms.

Summary:
This patch turns on `LIBCXX_ENABLE_ABI_LINKER_SCRIPT` by default whenever `LLVM_HAVE_LINK_VERSION_SCRIPT` is ON. This turns out to be whenever:

1. WIN32 is not defined.
2 UNIX is defined.
3. APPLE is not defined.

While `LLVM_HAVE_LINK_VERSION_SCRIPT` is meant to reflect exactly what we are asking I think it's close enough.

After committing this patch Linux users will no longer have to use "-lc++abi" explicitly!




Reviewers: mclow.lists, danalbert, compnerd, jroelofs

Subscribers: emaste, rengolin, cbergstrom, cfe-commits

Differential Revision: http://reviews.llvm.org/D13739

llvm-svn: 250469