[JITLink] Fix a pointer-to-integer cast in jitlink::InProcessMemoryManager.

reinterpret_cast'ing the block base address directly to a uint64_t leaves the
high bits in an implementation-defined state, but JITLink expects them to be
zero. Switching to pointerToJITTargetAddress for the cast should fix this.

This should fix the jitlink test failures that we have seen on some of the
32-bit testers.