[libunwind] Fix pointer-to-integer cast warnings on LLP64.

Summary:
`long` is too short on LLP64. We have to use `intptr_t` to
avoid truncating pointers.

Reviewers: mstorsjo, rnk, compnerd, smeenai

Subscribers: christof, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D50412

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@339217 91177308-0d34-0410-b5e6-96231b3b80d8