[libunwind] Use placement new to avoid dependency C++ library

The rest of libunwind already uses placement new, these are the only
places where non-placement new is being used introducing undesirable
C++ library dependency.

Differential Revision: https://reviews.llvm.org/D57251

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@352245 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed