[libc++abi] Do not define -Dcxxabi_shared_EXPORTS when building libc++abi

CMake sets adds that definition automatically, but we don't need
or use it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@373940 91177308-0d34-0410-b5e6-96231b3b80d8