[RuntimeDyld] Change the return type of decodeAddend to match the storage type.
[RuntimeDyld][MachO][AArch64] Add assertion to check for duplicate addend definition.

In MachO for AArch64 it is possible to have an explicit addend defined by
the ARM64_RELOC_ADDEND relocation or having an addend encoded within the
instruction. Only one of them are allowed per relocation.
[RuntimeDyld][MachO][AArch64] Implement the decodeAddend method.

This adds the required functionality to decode the immediate encoded in an
instruction that is referenced in a relocation entry.
[RuntimeDyld][MachO][AArch64] Add a helper function for encoding addends in instructions.

Factor out the addend encoding into a helper function and simplify the
processRelocationRef.

Also add a few simple rtdyld tests. More tests to come once GOTs can be tested too.

Related to <rdar://problem/17768539>