[JITLink] Add a -slab-address option to llvm-jitlink.

This option can be used to for JITLink to link as-if the target memory slab were
allocated at a specific start address. This can be used to both verify that
cross-address space linking is working correctly, and to ensure that certain
address-sensitive optimizations (e.g. GOT and stub elimination) either do or do
not fire, depending on the requirements of the test case.

This argument is only valid for testing in conjunction with -noexec -slab-alloc,
and will produce an error if used without those arguments.