In RuntimeDyldImpl::emitSection, make Allocate (section size to be allocated) a
uintptr_t. An unsigned could overflow for large sections.

No test case - anything big enough to overflow an unsigned is going to take an
appreciable time to zero when the test passes.

The choice of uintptr_t was made to match the RTDyldMemoryManager APIs, but
these should probably be hardcoded to uint64_ts: It is legitimate to JIT for
64-bit targets from a 32-bit host/compiler.