[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

llvm-svn: 339217