A64: Handle a few TODOs.

Below are notes about some of the TODOs handled.

* In Simulator::Debug:
    We haven't had issues with printing in the debugger for a while. If the
    feature turns out to be still lacking an issue can be raised.

* In SubStringStub::Generate:
    There is no need to jump there. The StringCharAtGenerator is asked to set
    the result in x0, and the return sequence following only uses two instructions.
    Also, we would need to jump after the native counter increment instead of the
    return_x0 label.

* In ArrayPushStub::Generate:
    Raised an internal issue to check and update the ARM code.

* In MacroAssembler::CopyBytes:
    The new code should be faster for bulk copies. Raised an internal issue to
    investigate and improve this function.

* In MacroAssembler::EnterExitFrame:
    Unless we actually find something is wrong, this is working and looks ok.
    Also fixed the comments about the FP registers.

* Comment for BumpSystemStackPointer:
    Removed the comment at the declaration site and updated the TODO in the
    implementation.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/196473021

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20034 ce2b1a6d-e550-0410-aec6-3dcde31c8c00