Fix warnings about printf format strings

Either adjust the format string to use a more exact type, or add casts
(for cases when printing pointers to structs/objects with a %p
format specifier).

Differential Revision: https://reviews.llvm.org/D56982

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@351876 91177308-0d34-0410-b5e6-96231b3b80d8