Fix unwinder to build for iOS

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@199859 91177308-0d34-0410-b5e6-96231b3b80d8
Don't redefine static_assert()

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@199860 91177308-0d34-0410-b5e6-96231b3b80d8