[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

llvm-svn: 352245
GitOrigin-RevId: 90bcfaa2a065bb43b2994c3eb3fa5775ef319e84
1 file changed