Use 64-bit pointer to unwind stack for x86-64

X32 uses ILP32 data model in 64-bit hardware mode.  This patch always
uses 64-bit pointer to unwind stack for x86-64.
Patch by H.J. Lu


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209277 91177308-0d34-0410-b5e6-96231b3b80d8