Revert "[libunwind] Drop the dependency on <algorithm>, add placement new inline"

This reverts commit r352384: this broke on ARM as UnwindCursor.hpp
still has some C++ library dependencies.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@352427 91177308-0d34-0410-b5e6-96231b3b80d8