A64: Handle a few TODOs.

Notes about a few TODOs handled in this patch:

* In ProfileEntryHookStub::Generate:
    Stubs are always called with relocation.

* In CreateArrayDispatchOneArgument:
    The branches to registers can't be conditional. We could use a jump table, but
    there are only 6 different kinds so it is likely not worth it.

* In Builtins::Generate_StringConstructCode:
    Rename the argc register (x0) after its meaning changes.
    Remove a TODO: using a macro would not make the code clearer.

* In Generate_JSEntryTrampolineHelper:
    Remove the TODO and raise an internal issue to investigate this.

* In Disassembler::SubstituteBranchTargetField:
    Print the target address, but we don't have more info on the target.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/185793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19724 ce2b1a6d-e550-0410-aec6-3dcde31c8c00