[RuntimeDyld, PowerPC] Fix relocation detection overflow

Code in RuntimeDyldELF currently uses 32-bit temporaries to detect
whether a PPC64 relocation target is out of range. This is incorrect,
and can mis-detect overflow where the distance between relocation site
and target is close to a multiple of 4GB. Fixed by using 64-bit
temporaries.

Noticed while debugging PR32650.

Reviewer: hfinkel

Differential Revision: https://reviews.llvm.org/D33403

llvm-svn: 303632