[libc++abi] Do not define -Dcxxabi_shared_EXPORTS when building libc++abi
[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: 6a9f6578bdee8d959f0ed58970538b4ab6004734@373940 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi