[MCJIT] Replace a c-style cast with reinterpret_cast + static_cast.

C-style casts (and reinterpret_casts) result in implementation defined
values when a pointer is cast to a larger integer type. On some platforms
this was leading to bogus address computations in RuntimeDyldMachOAArch64.

This should fix http://llvm.org/PR20501.

llvm-svn: 215143